Using Mean Value Theorem, Prove that ${tan xover x}>{xoversin x}$
$begingroup$
Prove using Mean Value Theorem,
$${tan xover x}>{xoversin x} spaceforall space x spaceepsilon (0, pi/2) $$
Attempt::
$f(x) = x-sin x$
$f'(x) = 1-cos x > 0 $
Hence $x-sin x>0, {xoversin x}>1$
Similarly, I got ${tan xover x}>1$
But how do I compare them and get the required inequality?
calculus limits inequality
$endgroup$
add a comment |
$begingroup$
Prove using Mean Value Theorem,
$${tan xover x}>{xoversin x} spaceforall space x spaceepsilon (0, pi/2) $$
Attempt::
$f(x) = x-sin x$
$f'(x) = 1-cos x > 0 $
Hence $x-sin x>0, {xoversin x}>1$
Similarly, I got ${tan xover x}>1$
But how do I compare them and get the required inequality?
calculus limits inequality
$endgroup$
$begingroup$
For $xin mathbb R$ or in some interval? (In $mathbb R$ it does not hold)
$endgroup$
– Jimmy R.
Dec 2 '16 at 14:45
$begingroup$
Isn't $x$ restricted to any interval?
$endgroup$
– ajotatxe
Dec 2 '16 at 14:45
1
$begingroup$
Prove $tanx/x >1$ and $sinx/x<1$ and any interval restriction ?
$endgroup$
– papabiceps
Dec 2 '16 at 14:45
1
$begingroup$
Interval is between 0<x<pi/2
$endgroup$
– user3442005
Dec 2 '16 at 15:05
add a comment |
$begingroup$
Prove using Mean Value Theorem,
$${tan xover x}>{xoversin x} spaceforall space x spaceepsilon (0, pi/2) $$
Attempt::
$f(x) = x-sin x$
$f'(x) = 1-cos x > 0 $
Hence $x-sin x>0, {xoversin x}>1$
Similarly, I got ${tan xover x}>1$
But how do I compare them and get the required inequality?
calculus limits inequality
$endgroup$
Prove using Mean Value Theorem,
$${tan xover x}>{xoversin x} spaceforall space x spaceepsilon (0, pi/2) $$
Attempt::
$f(x) = x-sin x$
$f'(x) = 1-cos x > 0 $
Hence $x-sin x>0, {xoversin x}>1$
Similarly, I got ${tan xover x}>1$
But how do I compare them and get the required inequality?
calculus limits inequality
calculus limits inequality
edited Dec 18 '18 at 5:58
tatan
5,79762759
5,79762759
asked Dec 2 '16 at 14:41
user3442005user3442005
305
305
$begingroup$
For $xin mathbb R$ or in some interval? (In $mathbb R$ it does not hold)
$endgroup$
– Jimmy R.
Dec 2 '16 at 14:45
$begingroup$
Isn't $x$ restricted to any interval?
$endgroup$
– ajotatxe
Dec 2 '16 at 14:45
1
$begingroup$
Prove $tanx/x >1$ and $sinx/x<1$ and any interval restriction ?
$endgroup$
– papabiceps
Dec 2 '16 at 14:45
1
$begingroup$
Interval is between 0<x<pi/2
$endgroup$
– user3442005
Dec 2 '16 at 15:05
add a comment |
$begingroup$
For $xin mathbb R$ or in some interval? (In $mathbb R$ it does not hold)
$endgroup$
– Jimmy R.
Dec 2 '16 at 14:45
$begingroup$
Isn't $x$ restricted to any interval?
$endgroup$
– ajotatxe
Dec 2 '16 at 14:45
1
$begingroup$
Prove $tanx/x >1$ and $sinx/x<1$ and any interval restriction ?
$endgroup$
– papabiceps
Dec 2 '16 at 14:45
1
$begingroup$
Interval is between 0<x<pi/2
$endgroup$
– user3442005
Dec 2 '16 at 15:05
$begingroup$
For $xin mathbb R$ or in some interval? (In $mathbb R$ it does not hold)
$endgroup$
– Jimmy R.
Dec 2 '16 at 14:45
$begingroup$
For $xin mathbb R$ or in some interval? (In $mathbb R$ it does not hold)
$endgroup$
– Jimmy R.
Dec 2 '16 at 14:45
$begingroup$
Isn't $x$ restricted to any interval?
$endgroup$
– ajotatxe
Dec 2 '16 at 14:45
$begingroup$
Isn't $x$ restricted to any interval?
$endgroup$
– ajotatxe
Dec 2 '16 at 14:45
1
1
$begingroup$
Prove $tanx/x >1$ and $sinx/x<1$ and any interval restriction ?
$endgroup$
– papabiceps
Dec 2 '16 at 14:45
$begingroup$
Prove $tanx/x >1$ and $sinx/x<1$ and any interval restriction ?
$endgroup$
– papabiceps
Dec 2 '16 at 14:45
1
1
$begingroup$
Interval is between 0<x<pi/2
$endgroup$
– user3442005
Dec 2 '16 at 15:05
$begingroup$
Interval is between 0<x<pi/2
$endgroup$
– user3442005
Dec 2 '16 at 15:05
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
We need to prove that $f(x)>0$, where $f(x)=frac{sin{x}}{sqrt{cos{x}}}-x$.
But $f'(x)=frac{cos xsqrt{cos x}+frac{sin^2x}{2sqrt{cos x}}}{cos{x}}-1=frac{(sqrt{cos^3{x}}-1)^2+cos^2x(1-cos{x})}{2sqrt{cos^3x}}>0$.
Thus, $f(x)>f(0)=0$.
Done!
$endgroup$
$begingroup$
How is f(x) = sinx/root(cosx) -x ?That isnt the question.
$endgroup$
– user3442005
Dec 2 '16 at 16:58
$begingroup$
@user3442005 We need to prove that $frac{sin^2x}{cos{x}}>x^2$, which is $f(x)>0$.
$endgroup$
– Michael Rozenberg
Dec 2 '16 at 17:27
$begingroup$
Where did you use Mean Value Theorem?
$endgroup$
– user261263
Dec 3 '16 at 6:05
add a comment |
$begingroup$
For each $x in (0,2pi),$ the MVT(mean value theorem) makes sure that there exists $c_x$ with $0 < c_x < x$ such that
$$frac{sin x}{x} = cos c_x < 1.$$
With $f(x) = sin x$ we have $sin x = sin x - sin 0 = f'(c_x)(x-0)= (cos c_x)x$
where $0 < c_x < x$.
Now for $displaystyle frac{tan x}{x}>1$
$f(x) = tan x$ then $f'(x) = sec^2(x)$. So $sec^2(x)$ = $displaystyle frac{tan x}{x}$
For each $x in (0,2pi)$ $sec^2(x) >1$. So $displaystyle frac{tan x}{x}$ $>1$.
$endgroup$
$begingroup$
I had forgotten to mention the limits, it is from (0, pi/2)
$endgroup$
– user3442005
Dec 2 '16 at 15:13
$begingroup$
Then my answer holds true for your given limits.
$endgroup$
– papabiceps
Dec 2 '16 at 15:15
$begingroup$
According to the solution, x/sinx>1 and tanx/x>1. How do I compare the two inequalities and get tanx/x > x/sinx?
$endgroup$
– user3442005
Dec 2 '16 at 15:24
$begingroup$
The comparison is with x/sin x, not sin x/x, so this does not solve the problem.
$endgroup$
– marty cohen
Dec 2 '16 at 15:30
$begingroup$
$tan$ function is not well defined on $x in (0,2pi)$
$endgroup$
– user261263
Dec 3 '16 at 6:04
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
We need to prove that $f(x)>0$, where $f(x)=frac{sin{x}}{sqrt{cos{x}}}-x$.
But $f'(x)=frac{cos xsqrt{cos x}+frac{sin^2x}{2sqrt{cos x}}}{cos{x}}-1=frac{(sqrt{cos^3{x}}-1)^2+cos^2x(1-cos{x})}{2sqrt{cos^3x}}>0$.
Thus, $f(x)>f(0)=0$.
Done!
$endgroup$
$begingroup$
How is f(x) = sinx/root(cosx) -x ?That isnt the question.
$endgroup$
– user3442005
Dec 2 '16 at 16:58
$begingroup$
@user3442005 We need to prove that $frac{sin^2x}{cos{x}}>x^2$, which is $f(x)>0$.
$endgroup$
– Michael Rozenberg
Dec 2 '16 at 17:27
$begingroup$
Where did you use Mean Value Theorem?
$endgroup$
– user261263
Dec 3 '16 at 6:05
add a comment |
$begingroup$
We need to prove that $f(x)>0$, where $f(x)=frac{sin{x}}{sqrt{cos{x}}}-x$.
But $f'(x)=frac{cos xsqrt{cos x}+frac{sin^2x}{2sqrt{cos x}}}{cos{x}}-1=frac{(sqrt{cos^3{x}}-1)^2+cos^2x(1-cos{x})}{2sqrt{cos^3x}}>0$.
Thus, $f(x)>f(0)=0$.
Done!
$endgroup$
$begingroup$
How is f(x) = sinx/root(cosx) -x ?That isnt the question.
$endgroup$
– user3442005
Dec 2 '16 at 16:58
$begingroup$
@user3442005 We need to prove that $frac{sin^2x}{cos{x}}>x^2$, which is $f(x)>0$.
$endgroup$
– Michael Rozenberg
Dec 2 '16 at 17:27
$begingroup$
Where did you use Mean Value Theorem?
$endgroup$
– user261263
Dec 3 '16 at 6:05
add a comment |
$begingroup$
We need to prove that $f(x)>0$, where $f(x)=frac{sin{x}}{sqrt{cos{x}}}-x$.
But $f'(x)=frac{cos xsqrt{cos x}+frac{sin^2x}{2sqrt{cos x}}}{cos{x}}-1=frac{(sqrt{cos^3{x}}-1)^2+cos^2x(1-cos{x})}{2sqrt{cos^3x}}>0$.
Thus, $f(x)>f(0)=0$.
Done!
$endgroup$
We need to prove that $f(x)>0$, where $f(x)=frac{sin{x}}{sqrt{cos{x}}}-x$.
But $f'(x)=frac{cos xsqrt{cos x}+frac{sin^2x}{2sqrt{cos x}}}{cos{x}}-1=frac{(sqrt{cos^3{x}}-1)^2+cos^2x(1-cos{x})}{2sqrt{cos^3x}}>0$.
Thus, $f(x)>f(0)=0$.
Done!
answered Dec 2 '16 at 16:44
Michael RozenbergMichael Rozenberg
105k1892198
105k1892198
$begingroup$
How is f(x) = sinx/root(cosx) -x ?That isnt the question.
$endgroup$
– user3442005
Dec 2 '16 at 16:58
$begingroup$
@user3442005 We need to prove that $frac{sin^2x}{cos{x}}>x^2$, which is $f(x)>0$.
$endgroup$
– Michael Rozenberg
Dec 2 '16 at 17:27
$begingroup$
Where did you use Mean Value Theorem?
$endgroup$
– user261263
Dec 3 '16 at 6:05
add a comment |
$begingroup$
How is f(x) = sinx/root(cosx) -x ?That isnt the question.
$endgroup$
– user3442005
Dec 2 '16 at 16:58
$begingroup$
@user3442005 We need to prove that $frac{sin^2x}{cos{x}}>x^2$, which is $f(x)>0$.
$endgroup$
– Michael Rozenberg
Dec 2 '16 at 17:27
$begingroup$
Where did you use Mean Value Theorem?
$endgroup$
– user261263
Dec 3 '16 at 6:05
$begingroup$
How is f(x) = sinx/root(cosx) -x ?That isnt the question.
$endgroup$
– user3442005
Dec 2 '16 at 16:58
$begingroup$
How is f(x) = sinx/root(cosx) -x ?That isnt the question.
$endgroup$
– user3442005
Dec 2 '16 at 16:58
$begingroup$
@user3442005 We need to prove that $frac{sin^2x}{cos{x}}>x^2$, which is $f(x)>0$.
$endgroup$
– Michael Rozenberg
Dec 2 '16 at 17:27
$begingroup$
@user3442005 We need to prove that $frac{sin^2x}{cos{x}}>x^2$, which is $f(x)>0$.
$endgroup$
– Michael Rozenberg
Dec 2 '16 at 17:27
$begingroup$
Where did you use Mean Value Theorem?
$endgroup$
– user261263
Dec 3 '16 at 6:05
$begingroup$
Where did you use Mean Value Theorem?
$endgroup$
– user261263
Dec 3 '16 at 6:05
add a comment |
$begingroup$
For each $x in (0,2pi),$ the MVT(mean value theorem) makes sure that there exists $c_x$ with $0 < c_x < x$ such that
$$frac{sin x}{x} = cos c_x < 1.$$
With $f(x) = sin x$ we have $sin x = sin x - sin 0 = f'(c_x)(x-0)= (cos c_x)x$
where $0 < c_x < x$.
Now for $displaystyle frac{tan x}{x}>1$
$f(x) = tan x$ then $f'(x) = sec^2(x)$. So $sec^2(x)$ = $displaystyle frac{tan x}{x}$
For each $x in (0,2pi)$ $sec^2(x) >1$. So $displaystyle frac{tan x}{x}$ $>1$.
$endgroup$
$begingroup$
I had forgotten to mention the limits, it is from (0, pi/2)
$endgroup$
– user3442005
Dec 2 '16 at 15:13
$begingroup$
Then my answer holds true for your given limits.
$endgroup$
– papabiceps
Dec 2 '16 at 15:15
$begingroup$
According to the solution, x/sinx>1 and tanx/x>1. How do I compare the two inequalities and get tanx/x > x/sinx?
$endgroup$
– user3442005
Dec 2 '16 at 15:24
$begingroup$
The comparison is with x/sin x, not sin x/x, so this does not solve the problem.
$endgroup$
– marty cohen
Dec 2 '16 at 15:30
$begingroup$
$tan$ function is not well defined on $x in (0,2pi)$
$endgroup$
– user261263
Dec 3 '16 at 6:04
add a comment |
$begingroup$
For each $x in (0,2pi),$ the MVT(mean value theorem) makes sure that there exists $c_x$ with $0 < c_x < x$ such that
$$frac{sin x}{x} = cos c_x < 1.$$
With $f(x) = sin x$ we have $sin x = sin x - sin 0 = f'(c_x)(x-0)= (cos c_x)x$
where $0 < c_x < x$.
Now for $displaystyle frac{tan x}{x}>1$
$f(x) = tan x$ then $f'(x) = sec^2(x)$. So $sec^2(x)$ = $displaystyle frac{tan x}{x}$
For each $x in (0,2pi)$ $sec^2(x) >1$. So $displaystyle frac{tan x}{x}$ $>1$.
$endgroup$
$begingroup$
I had forgotten to mention the limits, it is from (0, pi/2)
$endgroup$
– user3442005
Dec 2 '16 at 15:13
$begingroup$
Then my answer holds true for your given limits.
$endgroup$
– papabiceps
Dec 2 '16 at 15:15
$begingroup$
According to the solution, x/sinx>1 and tanx/x>1. How do I compare the two inequalities and get tanx/x > x/sinx?
$endgroup$
– user3442005
Dec 2 '16 at 15:24
$begingroup$
The comparison is with x/sin x, not sin x/x, so this does not solve the problem.
$endgroup$
– marty cohen
Dec 2 '16 at 15:30
$begingroup$
$tan$ function is not well defined on $x in (0,2pi)$
$endgroup$
– user261263
Dec 3 '16 at 6:04
add a comment |
$begingroup$
For each $x in (0,2pi),$ the MVT(mean value theorem) makes sure that there exists $c_x$ with $0 < c_x < x$ such that
$$frac{sin x}{x} = cos c_x < 1.$$
With $f(x) = sin x$ we have $sin x = sin x - sin 0 = f'(c_x)(x-0)= (cos c_x)x$
where $0 < c_x < x$.
Now for $displaystyle frac{tan x}{x}>1$
$f(x) = tan x$ then $f'(x) = sec^2(x)$. So $sec^2(x)$ = $displaystyle frac{tan x}{x}$
For each $x in (0,2pi)$ $sec^2(x) >1$. So $displaystyle frac{tan x}{x}$ $>1$.
$endgroup$
For each $x in (0,2pi),$ the MVT(mean value theorem) makes sure that there exists $c_x$ with $0 < c_x < x$ such that
$$frac{sin x}{x} = cos c_x < 1.$$
With $f(x) = sin x$ we have $sin x = sin x - sin 0 = f'(c_x)(x-0)= (cos c_x)x$
where $0 < c_x < x$.
Now for $displaystyle frac{tan x}{x}>1$
$f(x) = tan x$ then $f'(x) = sec^2(x)$. So $sec^2(x)$ = $displaystyle frac{tan x}{x}$
For each $x in (0,2pi)$ $sec^2(x) >1$. So $displaystyle frac{tan x}{x}$ $>1$.
edited Dec 2 '16 at 15:04
juantheron
34.3k1148143
34.3k1148143
answered Dec 2 '16 at 14:55
papabicepspapabiceps
322113
322113
$begingroup$
I had forgotten to mention the limits, it is from (0, pi/2)
$endgroup$
– user3442005
Dec 2 '16 at 15:13
$begingroup$
Then my answer holds true for your given limits.
$endgroup$
– papabiceps
Dec 2 '16 at 15:15
$begingroup$
According to the solution, x/sinx>1 and tanx/x>1. How do I compare the two inequalities and get tanx/x > x/sinx?
$endgroup$
– user3442005
Dec 2 '16 at 15:24
$begingroup$
The comparison is with x/sin x, not sin x/x, so this does not solve the problem.
$endgroup$
– marty cohen
Dec 2 '16 at 15:30
$begingroup$
$tan$ function is not well defined on $x in (0,2pi)$
$endgroup$
– user261263
Dec 3 '16 at 6:04
add a comment |
$begingroup$
I had forgotten to mention the limits, it is from (0, pi/2)
$endgroup$
– user3442005
Dec 2 '16 at 15:13
$begingroup$
Then my answer holds true for your given limits.
$endgroup$
– papabiceps
Dec 2 '16 at 15:15
$begingroup$
According to the solution, x/sinx>1 and tanx/x>1. How do I compare the two inequalities and get tanx/x > x/sinx?
$endgroup$
– user3442005
Dec 2 '16 at 15:24
$begingroup$
The comparison is with x/sin x, not sin x/x, so this does not solve the problem.
$endgroup$
– marty cohen
Dec 2 '16 at 15:30
$begingroup$
$tan$ function is not well defined on $x in (0,2pi)$
$endgroup$
– user261263
Dec 3 '16 at 6:04
$begingroup$
I had forgotten to mention the limits, it is from (0, pi/2)
$endgroup$
– user3442005
Dec 2 '16 at 15:13
$begingroup$
I had forgotten to mention the limits, it is from (0, pi/2)
$endgroup$
– user3442005
Dec 2 '16 at 15:13
$begingroup$
Then my answer holds true for your given limits.
$endgroup$
– papabiceps
Dec 2 '16 at 15:15
$begingroup$
Then my answer holds true for your given limits.
$endgroup$
– papabiceps
Dec 2 '16 at 15:15
$begingroup$
According to the solution, x/sinx>1 and tanx/x>1. How do I compare the two inequalities and get tanx/x > x/sinx?
$endgroup$
– user3442005
Dec 2 '16 at 15:24
$begingroup$
According to the solution, x/sinx>1 and tanx/x>1. How do I compare the two inequalities and get tanx/x > x/sinx?
$endgroup$
– user3442005
Dec 2 '16 at 15:24
$begingroup$
The comparison is with x/sin x, not sin x/x, so this does not solve the problem.
$endgroup$
– marty cohen
Dec 2 '16 at 15:30
$begingroup$
The comparison is with x/sin x, not sin x/x, so this does not solve the problem.
$endgroup$
– marty cohen
Dec 2 '16 at 15:30
$begingroup$
$tan$ function is not well defined on $x in (0,2pi)$
$endgroup$
– user261263
Dec 3 '16 at 6:04
$begingroup$
$tan$ function is not well defined on $x in (0,2pi)$
$endgroup$
– user261263
Dec 3 '16 at 6:04
add a comment |
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$begingroup$
For $xin mathbb R$ or in some interval? (In $mathbb R$ it does not hold)
$endgroup$
– Jimmy R.
Dec 2 '16 at 14:45
$begingroup$
Isn't $x$ restricted to any interval?
$endgroup$
– ajotatxe
Dec 2 '16 at 14:45
1
$begingroup$
Prove $tanx/x >1$ and $sinx/x<1$ and any interval restriction ?
$endgroup$
– papabiceps
Dec 2 '16 at 14:45
1
$begingroup$
Interval is between 0<x<pi/2
$endgroup$
– user3442005
Dec 2 '16 at 15:05