Evaluate the integral $int_{-1}^1 frac{1}{(2-x)sqrt{1-x^2}},dx$












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I started with the substitution $x=sin(theta)$ with $thetain(-frac{pi}{2},frac{pi}{2})$



And I got the integral $$int_{-pi/2}^{pi/2} frac{1}{2-sin(theta)},dtheta$$
which I am again not able to evaluate.



Even integration by parts is not working.










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  • $begingroup$
    symbolab.com/solver/step-by-step/…
    $endgroup$
    – ricky
    Dec 19 '18 at 10:06










  • $begingroup$
    I hope this helps
    $endgroup$
    – ricky
    Dec 19 '18 at 10:06
















1












$begingroup$


I started with the substitution $x=sin(theta)$ with $thetain(-frac{pi}{2},frac{pi}{2})$



And I got the integral $$int_{-pi/2}^{pi/2} frac{1}{2-sin(theta)},dtheta$$
which I am again not able to evaluate.



Even integration by parts is not working.










share|cite|improve this question











$endgroup$












  • $begingroup$
    symbolab.com/solver/step-by-step/…
    $endgroup$
    – ricky
    Dec 19 '18 at 10:06










  • $begingroup$
    I hope this helps
    $endgroup$
    – ricky
    Dec 19 '18 at 10:06














1












1








1


4



$begingroup$


I started with the substitution $x=sin(theta)$ with $thetain(-frac{pi}{2},frac{pi}{2})$



And I got the integral $$int_{-pi/2}^{pi/2} frac{1}{2-sin(theta)},dtheta$$
which I am again not able to evaluate.



Even integration by parts is not working.










share|cite|improve this question











$endgroup$




I started with the substitution $x=sin(theta)$ with $thetain(-frac{pi}{2},frac{pi}{2})$



And I got the integral $$int_{-pi/2}^{pi/2} frac{1}{2-sin(theta)},dtheta$$
which I am again not able to evaluate.



Even integration by parts is not working.







integration riemann-integration






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share|cite|improve this question













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edited Dec 19 '18 at 10:26









quiliup

1799




1799










asked Dec 19 '18 at 9:56









Vinay VarahabhotlaVinay Varahabhotla

537




537












  • $begingroup$
    symbolab.com/solver/step-by-step/…
    $endgroup$
    – ricky
    Dec 19 '18 at 10:06










  • $begingroup$
    I hope this helps
    $endgroup$
    – ricky
    Dec 19 '18 at 10:06


















  • $begingroup$
    symbolab.com/solver/step-by-step/…
    $endgroup$
    – ricky
    Dec 19 '18 at 10:06










  • $begingroup$
    I hope this helps
    $endgroup$
    – ricky
    Dec 19 '18 at 10:06
















$begingroup$
symbolab.com/solver/step-by-step/…
$endgroup$
– ricky
Dec 19 '18 at 10:06




$begingroup$
symbolab.com/solver/step-by-step/…
$endgroup$
– ricky
Dec 19 '18 at 10:06












$begingroup$
I hope this helps
$endgroup$
– ricky
Dec 19 '18 at 10:06




$begingroup$
I hope this helps
$endgroup$
– ricky
Dec 19 '18 at 10:06










4 Answers
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4












$begingroup$

Try using the substitution $displaystyle t = tan frac{theta}{2}$, this is a handy substitution to make when there are trigonometric functions that you cannot simplify very easily. In our case we have that,



$$frac{mathrm{d}t}{mathrm{d}theta} = frac{1}{2} sec^2frac{theta}{2} = frac{1}{2} left(1 + tan^2 frac{theta}{2} right) = frac{1}{2}(1 + t^2) Rightarrow frac{2mathrm{d}t}{1+t^2} = mathrm{d}theta $$



The usefulness of this substitution is that we can now use properties of a right angled triangle that has angle $theta /2$ to say that,



$$sin theta = 2sin frac{theta}{2} cos frac{theta}{2} = 2 frac{1}{sqrt{1+t^2}} frac{t}{sqrt{1+t^2}}= frac{2t}{1+t^2} $$



Putting this into our integral we have,



$$int_{-pi/2}^{pi/2} frac{mathrm{d}theta}{2 - sin theta} = int_{-1}^1 frac{2}{1+t^2} cdot frac{1}{2 - frac{2t}{1+t^2}} mathrm{d}t = int_{-1}^1 frac{1}{t^2 - t+ 1} mathrm{d}t $$



Now we have no more square roots, we have an irreducible quadratic at the bottom so we can complete the square to obtain,



$$int_{-1}^1 frac{1}{t^2 - t + 1} mathrm{d} t = int_{-1}^1 frac{1}{(t - frac{1}{2})^2 + frac{3}{4}} mathrm{d}t = frac{2}{sqrt{3}} tan^{-1} left(frac{2}{sqrt{3}}left(t - frac{1}{2}right) right) Big|_{-1}^1 = frac{pi}{sqrt{3}} $$






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  • $begingroup$
    Thx very much . Substitution is very helpful
    $endgroup$
    – Vinay Varahabhotla
    Dec 19 '18 at 10:12










  • $begingroup$
    @VinayVarahabhotla That substitution is called the Tangent-half angle substitution, and is REALLY useful
    $endgroup$
    – clathratus
    Dec 19 '18 at 17:09



















0












$begingroup$

It is an integral of form $displaystyleint dfrac1{L sqrt{Q}}$, where L is a linear and Q is a quadratic. You could solve such integrals fairly easily with $L = 1/t$. Using this on your integral gives $displaystyle int dfrac{1}{sqrt{frac13 - (tsqrt 3 - frac2{sqrt{3}})^2}} dt$.



Not very appealing here but can be useful for other integrals of this type.






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    0












    $begingroup$

    Here is a slightly different approach.



    Let
    $$I = int_{-1}^1 frac{dx}{(2 - x) sqrt{1 - x^2}} qquad (1)$$
    Enforcing a substitution of $x mapsto - x$ gives
    $$I = int_{-1}^1 frac{dx}{(2 + x) sqrt{1 - x^2}} qquad (2)$$
    Adding (1) to (2) one obtains
    $$I = 2 int_{-1}^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}} = 4 int_0^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}},$$
    since the integrand is even between symmetric limits.



    Now set $x = sin theta$. As $dx = cos theta , dtheta$, doing so yields
    $$I = 4 int_0^{pi/2} frac{dtheta}{4 - sin^2 theta}.$$
    Taking advantage of the identity $sin^2 theta + cos^2 theta = 1$, the dominator in the above integral can be rewritten as
    $$I = 4 int_0^{pi/2} frac{dtheta}{3 sin^2 theta + 4 cos^2 theta} = 4 int_0^{pi/2} frac{sec^2 theta}{3 tan^2 theta + 4} , dtheta.$$
    Setting $u = tan theta, du = sec^2 theta , dtheta$ one has
    begin{align}
    I &= 4 int_0^infty frac{du}{3u^2 + 4}\
    &= frac{4}{3} int_0^infty frac{du}{u^2 + left (frac{2}{sqrt{3}} right )^2}\
    &= frac{4}{3} left [frac{sqrt{3}}{2} tan^{-1} left (frac{u sqrt{3}}{2} right ) right ]_0^infty\
    &= frac{pi}{sqrt{3}}.
    end{align}






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      $begingroup$

      Here's a cool generalization consider the integral
      $$J=I(x_1,x_2;a,b)=int_{x_1}^{x_2}frac{dx}{a+bsin x}$$
      We may preform a tangent-half-angle substitution:
      $$t=tan(x/2)Rightarrow dx=frac{2dt}{1+t^2}\Rightarrow sin x=frac{2t}{1+t^2}$$
      Thus
      $$J=2int_{t_1}^{t_2}frac{1}{a+2bfrac{t}{1+t^2}}frac{dt}{1+t^2}$$
      Where $t_1=tan(x_1/2)$ and $t_2=tan(x_2/2)$. Anyway after a little algebra,
      $$J=2int_{t_1}^{t_2}frac{dt}{at^2+2bt+a}$$
      Then we complete the square in the denominator:
      $$J=2int_{t_1}^{t_2}frac{dt}{a(t+frac{b}a)^2+a-frac{b^2}a}$$
      then setting $g=a-frac{b^2}a$ and preforming the substitution
      $$t+frac{b}a=sqrt{frac{g}a}tan uRightarrow dt=sqrt{frac{g}a}sec^2u, du$$
      we see that
      $$J=2sqrt{frac{g}a}int_{u_1}^{u_2}frac{sec^2u}{gtan^2u+g}du$$
      Where $u_1=arctanbigg[sqrt{frac{a}g}bigg(t_1+frac{a}bbigg)bigg]$ and $u_2=arctanbigg[sqrt{frac{a}g}bigg(t_2+frac{a}bbigg)bigg]$. After we note that $frac{sec^2u}{gtan^2u+g}=frac1g$, we see that
      $$J=frac2{sqrt{ag}}(u_2-u_1)$$
      And after a bunch of algebra
      $$I(x_1,x_2;a,b)=frac1{sqrt{a^2-b^2}}bigg[arctanfrac{atan(x_2/2)+b}{sqrt{a^2-b^2}}-arctanfrac{atan(x_1/2)+b}{sqrt{a^2-b^2}}bigg]$$
      Which works as long as $a^2>b^2$.






      share|cite|improve this answer









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        4 Answers
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        active

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        4 Answers
        4






        active

        oldest

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        active

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        active

        oldest

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        4












        $begingroup$

        Try using the substitution $displaystyle t = tan frac{theta}{2}$, this is a handy substitution to make when there are trigonometric functions that you cannot simplify very easily. In our case we have that,



        $$frac{mathrm{d}t}{mathrm{d}theta} = frac{1}{2} sec^2frac{theta}{2} = frac{1}{2} left(1 + tan^2 frac{theta}{2} right) = frac{1}{2}(1 + t^2) Rightarrow frac{2mathrm{d}t}{1+t^2} = mathrm{d}theta $$



        The usefulness of this substitution is that we can now use properties of a right angled triangle that has angle $theta /2$ to say that,



        $$sin theta = 2sin frac{theta}{2} cos frac{theta}{2} = 2 frac{1}{sqrt{1+t^2}} frac{t}{sqrt{1+t^2}}= frac{2t}{1+t^2} $$



        Putting this into our integral we have,



        $$int_{-pi/2}^{pi/2} frac{mathrm{d}theta}{2 - sin theta} = int_{-1}^1 frac{2}{1+t^2} cdot frac{1}{2 - frac{2t}{1+t^2}} mathrm{d}t = int_{-1}^1 frac{1}{t^2 - t+ 1} mathrm{d}t $$



        Now we have no more square roots, we have an irreducible quadratic at the bottom so we can complete the square to obtain,



        $$int_{-1}^1 frac{1}{t^2 - t + 1} mathrm{d} t = int_{-1}^1 frac{1}{(t - frac{1}{2})^2 + frac{3}{4}} mathrm{d}t = frac{2}{sqrt{3}} tan^{-1} left(frac{2}{sqrt{3}}left(t - frac{1}{2}right) right) Big|_{-1}^1 = frac{pi}{sqrt{3}} $$






        share|cite|improve this answer









        $endgroup$













        • $begingroup$
          Thx very much . Substitution is very helpful
          $endgroup$
          – Vinay Varahabhotla
          Dec 19 '18 at 10:12










        • $begingroup$
          @VinayVarahabhotla That substitution is called the Tangent-half angle substitution, and is REALLY useful
          $endgroup$
          – clathratus
          Dec 19 '18 at 17:09
















        4












        $begingroup$

        Try using the substitution $displaystyle t = tan frac{theta}{2}$, this is a handy substitution to make when there are trigonometric functions that you cannot simplify very easily. In our case we have that,



        $$frac{mathrm{d}t}{mathrm{d}theta} = frac{1}{2} sec^2frac{theta}{2} = frac{1}{2} left(1 + tan^2 frac{theta}{2} right) = frac{1}{2}(1 + t^2) Rightarrow frac{2mathrm{d}t}{1+t^2} = mathrm{d}theta $$



        The usefulness of this substitution is that we can now use properties of a right angled triangle that has angle $theta /2$ to say that,



        $$sin theta = 2sin frac{theta}{2} cos frac{theta}{2} = 2 frac{1}{sqrt{1+t^2}} frac{t}{sqrt{1+t^2}}= frac{2t}{1+t^2} $$



        Putting this into our integral we have,



        $$int_{-pi/2}^{pi/2} frac{mathrm{d}theta}{2 - sin theta} = int_{-1}^1 frac{2}{1+t^2} cdot frac{1}{2 - frac{2t}{1+t^2}} mathrm{d}t = int_{-1}^1 frac{1}{t^2 - t+ 1} mathrm{d}t $$



        Now we have no more square roots, we have an irreducible quadratic at the bottom so we can complete the square to obtain,



        $$int_{-1}^1 frac{1}{t^2 - t + 1} mathrm{d} t = int_{-1}^1 frac{1}{(t - frac{1}{2})^2 + frac{3}{4}} mathrm{d}t = frac{2}{sqrt{3}} tan^{-1} left(frac{2}{sqrt{3}}left(t - frac{1}{2}right) right) Big|_{-1}^1 = frac{pi}{sqrt{3}} $$






        share|cite|improve this answer









        $endgroup$













        • $begingroup$
          Thx very much . Substitution is very helpful
          $endgroup$
          – Vinay Varahabhotla
          Dec 19 '18 at 10:12










        • $begingroup$
          @VinayVarahabhotla That substitution is called the Tangent-half angle substitution, and is REALLY useful
          $endgroup$
          – clathratus
          Dec 19 '18 at 17:09














        4












        4








        4





        $begingroup$

        Try using the substitution $displaystyle t = tan frac{theta}{2}$, this is a handy substitution to make when there are trigonometric functions that you cannot simplify very easily. In our case we have that,



        $$frac{mathrm{d}t}{mathrm{d}theta} = frac{1}{2} sec^2frac{theta}{2} = frac{1}{2} left(1 + tan^2 frac{theta}{2} right) = frac{1}{2}(1 + t^2) Rightarrow frac{2mathrm{d}t}{1+t^2} = mathrm{d}theta $$



        The usefulness of this substitution is that we can now use properties of a right angled triangle that has angle $theta /2$ to say that,



        $$sin theta = 2sin frac{theta}{2} cos frac{theta}{2} = 2 frac{1}{sqrt{1+t^2}} frac{t}{sqrt{1+t^2}}= frac{2t}{1+t^2} $$



        Putting this into our integral we have,



        $$int_{-pi/2}^{pi/2} frac{mathrm{d}theta}{2 - sin theta} = int_{-1}^1 frac{2}{1+t^2} cdot frac{1}{2 - frac{2t}{1+t^2}} mathrm{d}t = int_{-1}^1 frac{1}{t^2 - t+ 1} mathrm{d}t $$



        Now we have no more square roots, we have an irreducible quadratic at the bottom so we can complete the square to obtain,



        $$int_{-1}^1 frac{1}{t^2 - t + 1} mathrm{d} t = int_{-1}^1 frac{1}{(t - frac{1}{2})^2 + frac{3}{4}} mathrm{d}t = frac{2}{sqrt{3}} tan^{-1} left(frac{2}{sqrt{3}}left(t - frac{1}{2}right) right) Big|_{-1}^1 = frac{pi}{sqrt{3}} $$






        share|cite|improve this answer









        $endgroup$



        Try using the substitution $displaystyle t = tan frac{theta}{2}$, this is a handy substitution to make when there are trigonometric functions that you cannot simplify very easily. In our case we have that,



        $$frac{mathrm{d}t}{mathrm{d}theta} = frac{1}{2} sec^2frac{theta}{2} = frac{1}{2} left(1 + tan^2 frac{theta}{2} right) = frac{1}{2}(1 + t^2) Rightarrow frac{2mathrm{d}t}{1+t^2} = mathrm{d}theta $$



        The usefulness of this substitution is that we can now use properties of a right angled triangle that has angle $theta /2$ to say that,



        $$sin theta = 2sin frac{theta}{2} cos frac{theta}{2} = 2 frac{1}{sqrt{1+t^2}} frac{t}{sqrt{1+t^2}}= frac{2t}{1+t^2} $$



        Putting this into our integral we have,



        $$int_{-pi/2}^{pi/2} frac{mathrm{d}theta}{2 - sin theta} = int_{-1}^1 frac{2}{1+t^2} cdot frac{1}{2 - frac{2t}{1+t^2}} mathrm{d}t = int_{-1}^1 frac{1}{t^2 - t+ 1} mathrm{d}t $$



        Now we have no more square roots, we have an irreducible quadratic at the bottom so we can complete the square to obtain,



        $$int_{-1}^1 frac{1}{t^2 - t + 1} mathrm{d} t = int_{-1}^1 frac{1}{(t - frac{1}{2})^2 + frac{3}{4}} mathrm{d}t = frac{2}{sqrt{3}} tan^{-1} left(frac{2}{sqrt{3}}left(t - frac{1}{2}right) right) Big|_{-1}^1 = frac{pi}{sqrt{3}} $$







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        share|cite|improve this answer










        answered Dec 19 '18 at 10:10









        symchdmathsymchdmath

        56817




        56817












        • $begingroup$
          Thx very much . Substitution is very helpful
          $endgroup$
          – Vinay Varahabhotla
          Dec 19 '18 at 10:12










        • $begingroup$
          @VinayVarahabhotla That substitution is called the Tangent-half angle substitution, and is REALLY useful
          $endgroup$
          – clathratus
          Dec 19 '18 at 17:09


















        • $begingroup$
          Thx very much . Substitution is very helpful
          $endgroup$
          – Vinay Varahabhotla
          Dec 19 '18 at 10:12










        • $begingroup$
          @VinayVarahabhotla That substitution is called the Tangent-half angle substitution, and is REALLY useful
          $endgroup$
          – clathratus
          Dec 19 '18 at 17:09
















        $begingroup$
        Thx very much . Substitution is very helpful
        $endgroup$
        – Vinay Varahabhotla
        Dec 19 '18 at 10:12




        $begingroup$
        Thx very much . Substitution is very helpful
        $endgroup$
        – Vinay Varahabhotla
        Dec 19 '18 at 10:12












        $begingroup$
        @VinayVarahabhotla That substitution is called the Tangent-half angle substitution, and is REALLY useful
        $endgroup$
        – clathratus
        Dec 19 '18 at 17:09




        $begingroup$
        @VinayVarahabhotla That substitution is called the Tangent-half angle substitution, and is REALLY useful
        $endgroup$
        – clathratus
        Dec 19 '18 at 17:09











        0












        $begingroup$

        It is an integral of form $displaystyleint dfrac1{L sqrt{Q}}$, where L is a linear and Q is a quadratic. You could solve such integrals fairly easily with $L = 1/t$. Using this on your integral gives $displaystyle int dfrac{1}{sqrt{frac13 - (tsqrt 3 - frac2{sqrt{3}})^2}} dt$.



        Not very appealing here but can be useful for other integrals of this type.






        share|cite|improve this answer









        $endgroup$


















          0












          $begingroup$

          It is an integral of form $displaystyleint dfrac1{L sqrt{Q}}$, where L is a linear and Q is a quadratic. You could solve such integrals fairly easily with $L = 1/t$. Using this on your integral gives $displaystyle int dfrac{1}{sqrt{frac13 - (tsqrt 3 - frac2{sqrt{3}})^2}} dt$.



          Not very appealing here but can be useful for other integrals of this type.






          share|cite|improve this answer









          $endgroup$
















            0












            0








            0





            $begingroup$

            It is an integral of form $displaystyleint dfrac1{L sqrt{Q}}$, where L is a linear and Q is a quadratic. You could solve such integrals fairly easily with $L = 1/t$. Using this on your integral gives $displaystyle int dfrac{1}{sqrt{frac13 - (tsqrt 3 - frac2{sqrt{3}})^2}} dt$.



            Not very appealing here but can be useful for other integrals of this type.






            share|cite|improve this answer









            $endgroup$



            It is an integral of form $displaystyleint dfrac1{L sqrt{Q}}$, where L is a linear and Q is a quadratic. You could solve such integrals fairly easily with $L = 1/t$. Using this on your integral gives $displaystyle int dfrac{1}{sqrt{frac13 - (tsqrt 3 - frac2{sqrt{3}})^2}} dt$.



            Not very appealing here but can be useful for other integrals of this type.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Dec 19 '18 at 14:58









            user8277998user8277998

            1,691521




            1,691521























                0












                $begingroup$

                Here is a slightly different approach.



                Let
                $$I = int_{-1}^1 frac{dx}{(2 - x) sqrt{1 - x^2}} qquad (1)$$
                Enforcing a substitution of $x mapsto - x$ gives
                $$I = int_{-1}^1 frac{dx}{(2 + x) sqrt{1 - x^2}} qquad (2)$$
                Adding (1) to (2) one obtains
                $$I = 2 int_{-1}^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}} = 4 int_0^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}},$$
                since the integrand is even between symmetric limits.



                Now set $x = sin theta$. As $dx = cos theta , dtheta$, doing so yields
                $$I = 4 int_0^{pi/2} frac{dtheta}{4 - sin^2 theta}.$$
                Taking advantage of the identity $sin^2 theta + cos^2 theta = 1$, the dominator in the above integral can be rewritten as
                $$I = 4 int_0^{pi/2} frac{dtheta}{3 sin^2 theta + 4 cos^2 theta} = 4 int_0^{pi/2} frac{sec^2 theta}{3 tan^2 theta + 4} , dtheta.$$
                Setting $u = tan theta, du = sec^2 theta , dtheta$ one has
                begin{align}
                I &= 4 int_0^infty frac{du}{3u^2 + 4}\
                &= frac{4}{3} int_0^infty frac{du}{u^2 + left (frac{2}{sqrt{3}} right )^2}\
                &= frac{4}{3} left [frac{sqrt{3}}{2} tan^{-1} left (frac{u sqrt{3}}{2} right ) right ]_0^infty\
                &= frac{pi}{sqrt{3}}.
                end{align}






                share|cite|improve this answer









                $endgroup$


















                  0












                  $begingroup$

                  Here is a slightly different approach.



                  Let
                  $$I = int_{-1}^1 frac{dx}{(2 - x) sqrt{1 - x^2}} qquad (1)$$
                  Enforcing a substitution of $x mapsto - x$ gives
                  $$I = int_{-1}^1 frac{dx}{(2 + x) sqrt{1 - x^2}} qquad (2)$$
                  Adding (1) to (2) one obtains
                  $$I = 2 int_{-1}^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}} = 4 int_0^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}},$$
                  since the integrand is even between symmetric limits.



                  Now set $x = sin theta$. As $dx = cos theta , dtheta$, doing so yields
                  $$I = 4 int_0^{pi/2} frac{dtheta}{4 - sin^2 theta}.$$
                  Taking advantage of the identity $sin^2 theta + cos^2 theta = 1$, the dominator in the above integral can be rewritten as
                  $$I = 4 int_0^{pi/2} frac{dtheta}{3 sin^2 theta + 4 cos^2 theta} = 4 int_0^{pi/2} frac{sec^2 theta}{3 tan^2 theta + 4} , dtheta.$$
                  Setting $u = tan theta, du = sec^2 theta , dtheta$ one has
                  begin{align}
                  I &= 4 int_0^infty frac{du}{3u^2 + 4}\
                  &= frac{4}{3} int_0^infty frac{du}{u^2 + left (frac{2}{sqrt{3}} right )^2}\
                  &= frac{4}{3} left [frac{sqrt{3}}{2} tan^{-1} left (frac{u sqrt{3}}{2} right ) right ]_0^infty\
                  &= frac{pi}{sqrt{3}}.
                  end{align}






                  share|cite|improve this answer









                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    Here is a slightly different approach.



                    Let
                    $$I = int_{-1}^1 frac{dx}{(2 - x) sqrt{1 - x^2}} qquad (1)$$
                    Enforcing a substitution of $x mapsto - x$ gives
                    $$I = int_{-1}^1 frac{dx}{(2 + x) sqrt{1 - x^2}} qquad (2)$$
                    Adding (1) to (2) one obtains
                    $$I = 2 int_{-1}^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}} = 4 int_0^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}},$$
                    since the integrand is even between symmetric limits.



                    Now set $x = sin theta$. As $dx = cos theta , dtheta$, doing so yields
                    $$I = 4 int_0^{pi/2} frac{dtheta}{4 - sin^2 theta}.$$
                    Taking advantage of the identity $sin^2 theta + cos^2 theta = 1$, the dominator in the above integral can be rewritten as
                    $$I = 4 int_0^{pi/2} frac{dtheta}{3 sin^2 theta + 4 cos^2 theta} = 4 int_0^{pi/2} frac{sec^2 theta}{3 tan^2 theta + 4} , dtheta.$$
                    Setting $u = tan theta, du = sec^2 theta , dtheta$ one has
                    begin{align}
                    I &= 4 int_0^infty frac{du}{3u^2 + 4}\
                    &= frac{4}{3} int_0^infty frac{du}{u^2 + left (frac{2}{sqrt{3}} right )^2}\
                    &= frac{4}{3} left [frac{sqrt{3}}{2} tan^{-1} left (frac{u sqrt{3}}{2} right ) right ]_0^infty\
                    &= frac{pi}{sqrt{3}}.
                    end{align}






                    share|cite|improve this answer









                    $endgroup$



                    Here is a slightly different approach.



                    Let
                    $$I = int_{-1}^1 frac{dx}{(2 - x) sqrt{1 - x^2}} qquad (1)$$
                    Enforcing a substitution of $x mapsto - x$ gives
                    $$I = int_{-1}^1 frac{dx}{(2 + x) sqrt{1 - x^2}} qquad (2)$$
                    Adding (1) to (2) one obtains
                    $$I = 2 int_{-1}^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}} = 4 int_0^1 frac{dx}{(4 - x^2) sqrt{1 - x^2}},$$
                    since the integrand is even between symmetric limits.



                    Now set $x = sin theta$. As $dx = cos theta , dtheta$, doing so yields
                    $$I = 4 int_0^{pi/2} frac{dtheta}{4 - sin^2 theta}.$$
                    Taking advantage of the identity $sin^2 theta + cos^2 theta = 1$, the dominator in the above integral can be rewritten as
                    $$I = 4 int_0^{pi/2} frac{dtheta}{3 sin^2 theta + 4 cos^2 theta} = 4 int_0^{pi/2} frac{sec^2 theta}{3 tan^2 theta + 4} , dtheta.$$
                    Setting $u = tan theta, du = sec^2 theta , dtheta$ one has
                    begin{align}
                    I &= 4 int_0^infty frac{du}{3u^2 + 4}\
                    &= frac{4}{3} int_0^infty frac{du}{u^2 + left (frac{2}{sqrt{3}} right )^2}\
                    &= frac{4}{3} left [frac{sqrt{3}}{2} tan^{-1} left (frac{u sqrt{3}}{2} right ) right ]_0^infty\
                    &= frac{pi}{sqrt{3}}.
                    end{align}







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Dec 20 '18 at 2:53









                    omegadotomegadot

                    6,2822828




                    6,2822828























                        0












                        $begingroup$

                        Here's a cool generalization consider the integral
                        $$J=I(x_1,x_2;a,b)=int_{x_1}^{x_2}frac{dx}{a+bsin x}$$
                        We may preform a tangent-half-angle substitution:
                        $$t=tan(x/2)Rightarrow dx=frac{2dt}{1+t^2}\Rightarrow sin x=frac{2t}{1+t^2}$$
                        Thus
                        $$J=2int_{t_1}^{t_2}frac{1}{a+2bfrac{t}{1+t^2}}frac{dt}{1+t^2}$$
                        Where $t_1=tan(x_1/2)$ and $t_2=tan(x_2/2)$. Anyway after a little algebra,
                        $$J=2int_{t_1}^{t_2}frac{dt}{at^2+2bt+a}$$
                        Then we complete the square in the denominator:
                        $$J=2int_{t_1}^{t_2}frac{dt}{a(t+frac{b}a)^2+a-frac{b^2}a}$$
                        then setting $g=a-frac{b^2}a$ and preforming the substitution
                        $$t+frac{b}a=sqrt{frac{g}a}tan uRightarrow dt=sqrt{frac{g}a}sec^2u, du$$
                        we see that
                        $$J=2sqrt{frac{g}a}int_{u_1}^{u_2}frac{sec^2u}{gtan^2u+g}du$$
                        Where $u_1=arctanbigg[sqrt{frac{a}g}bigg(t_1+frac{a}bbigg)bigg]$ and $u_2=arctanbigg[sqrt{frac{a}g}bigg(t_2+frac{a}bbigg)bigg]$. After we note that $frac{sec^2u}{gtan^2u+g}=frac1g$, we see that
                        $$J=frac2{sqrt{ag}}(u_2-u_1)$$
                        And after a bunch of algebra
                        $$I(x_1,x_2;a,b)=frac1{sqrt{a^2-b^2}}bigg[arctanfrac{atan(x_2/2)+b}{sqrt{a^2-b^2}}-arctanfrac{atan(x_1/2)+b}{sqrt{a^2-b^2}}bigg]$$
                        Which works as long as $a^2>b^2$.






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          Here's a cool generalization consider the integral
                          $$J=I(x_1,x_2;a,b)=int_{x_1}^{x_2}frac{dx}{a+bsin x}$$
                          We may preform a tangent-half-angle substitution:
                          $$t=tan(x/2)Rightarrow dx=frac{2dt}{1+t^2}\Rightarrow sin x=frac{2t}{1+t^2}$$
                          Thus
                          $$J=2int_{t_1}^{t_2}frac{1}{a+2bfrac{t}{1+t^2}}frac{dt}{1+t^2}$$
                          Where $t_1=tan(x_1/2)$ and $t_2=tan(x_2/2)$. Anyway after a little algebra,
                          $$J=2int_{t_1}^{t_2}frac{dt}{at^2+2bt+a}$$
                          Then we complete the square in the denominator:
                          $$J=2int_{t_1}^{t_2}frac{dt}{a(t+frac{b}a)^2+a-frac{b^2}a}$$
                          then setting $g=a-frac{b^2}a$ and preforming the substitution
                          $$t+frac{b}a=sqrt{frac{g}a}tan uRightarrow dt=sqrt{frac{g}a}sec^2u, du$$
                          we see that
                          $$J=2sqrt{frac{g}a}int_{u_1}^{u_2}frac{sec^2u}{gtan^2u+g}du$$
                          Where $u_1=arctanbigg[sqrt{frac{a}g}bigg(t_1+frac{a}bbigg)bigg]$ and $u_2=arctanbigg[sqrt{frac{a}g}bigg(t_2+frac{a}bbigg)bigg]$. After we note that $frac{sec^2u}{gtan^2u+g}=frac1g$, we see that
                          $$J=frac2{sqrt{ag}}(u_2-u_1)$$
                          And after a bunch of algebra
                          $$I(x_1,x_2;a,b)=frac1{sqrt{a^2-b^2}}bigg[arctanfrac{atan(x_2/2)+b}{sqrt{a^2-b^2}}-arctanfrac{atan(x_1/2)+b}{sqrt{a^2-b^2}}bigg]$$
                          Which works as long as $a^2>b^2$.






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            Here's a cool generalization consider the integral
                            $$J=I(x_1,x_2;a,b)=int_{x_1}^{x_2}frac{dx}{a+bsin x}$$
                            We may preform a tangent-half-angle substitution:
                            $$t=tan(x/2)Rightarrow dx=frac{2dt}{1+t^2}\Rightarrow sin x=frac{2t}{1+t^2}$$
                            Thus
                            $$J=2int_{t_1}^{t_2}frac{1}{a+2bfrac{t}{1+t^2}}frac{dt}{1+t^2}$$
                            Where $t_1=tan(x_1/2)$ and $t_2=tan(x_2/2)$. Anyway after a little algebra,
                            $$J=2int_{t_1}^{t_2}frac{dt}{at^2+2bt+a}$$
                            Then we complete the square in the denominator:
                            $$J=2int_{t_1}^{t_2}frac{dt}{a(t+frac{b}a)^2+a-frac{b^2}a}$$
                            then setting $g=a-frac{b^2}a$ and preforming the substitution
                            $$t+frac{b}a=sqrt{frac{g}a}tan uRightarrow dt=sqrt{frac{g}a}sec^2u, du$$
                            we see that
                            $$J=2sqrt{frac{g}a}int_{u_1}^{u_2}frac{sec^2u}{gtan^2u+g}du$$
                            Where $u_1=arctanbigg[sqrt{frac{a}g}bigg(t_1+frac{a}bbigg)bigg]$ and $u_2=arctanbigg[sqrt{frac{a}g}bigg(t_2+frac{a}bbigg)bigg]$. After we note that $frac{sec^2u}{gtan^2u+g}=frac1g$, we see that
                            $$J=frac2{sqrt{ag}}(u_2-u_1)$$
                            And after a bunch of algebra
                            $$I(x_1,x_2;a,b)=frac1{sqrt{a^2-b^2}}bigg[arctanfrac{atan(x_2/2)+b}{sqrt{a^2-b^2}}-arctanfrac{atan(x_1/2)+b}{sqrt{a^2-b^2}}bigg]$$
                            Which works as long as $a^2>b^2$.






                            share|cite|improve this answer









                            $endgroup$



                            Here's a cool generalization consider the integral
                            $$J=I(x_1,x_2;a,b)=int_{x_1}^{x_2}frac{dx}{a+bsin x}$$
                            We may preform a tangent-half-angle substitution:
                            $$t=tan(x/2)Rightarrow dx=frac{2dt}{1+t^2}\Rightarrow sin x=frac{2t}{1+t^2}$$
                            Thus
                            $$J=2int_{t_1}^{t_2}frac{1}{a+2bfrac{t}{1+t^2}}frac{dt}{1+t^2}$$
                            Where $t_1=tan(x_1/2)$ and $t_2=tan(x_2/2)$. Anyway after a little algebra,
                            $$J=2int_{t_1}^{t_2}frac{dt}{at^2+2bt+a}$$
                            Then we complete the square in the denominator:
                            $$J=2int_{t_1}^{t_2}frac{dt}{a(t+frac{b}a)^2+a-frac{b^2}a}$$
                            then setting $g=a-frac{b^2}a$ and preforming the substitution
                            $$t+frac{b}a=sqrt{frac{g}a}tan uRightarrow dt=sqrt{frac{g}a}sec^2u, du$$
                            we see that
                            $$J=2sqrt{frac{g}a}int_{u_1}^{u_2}frac{sec^2u}{gtan^2u+g}du$$
                            Where $u_1=arctanbigg[sqrt{frac{a}g}bigg(t_1+frac{a}bbigg)bigg]$ and $u_2=arctanbigg[sqrt{frac{a}g}bigg(t_2+frac{a}bbigg)bigg]$. After we note that $frac{sec^2u}{gtan^2u+g}=frac1g$, we see that
                            $$J=frac2{sqrt{ag}}(u_2-u_1)$$
                            And after a bunch of algebra
                            $$I(x_1,x_2;a,b)=frac1{sqrt{a^2-b^2}}bigg[arctanfrac{atan(x_2/2)+b}{sqrt{a^2-b^2}}-arctanfrac{atan(x_1/2)+b}{sqrt{a^2-b^2}}bigg]$$
                            Which works as long as $a^2>b^2$.







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Jan 12 at 4:45









                            clathratusclathratus

                            4,725337




                            4,725337






























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