Simplification of $cos^4(x) + sin^4(x)$
$begingroup$
$$begin{align}
(sin x)^4+(cos x)^4
&= (1-cos2x)^2/4 + (1+cos2x)^2/4\
&= (1-2cos2x+(cos2x)^2+1+2cos2x+(cos2x)^2)/4\
&= (1+(cos2x)^2)/2\
end{align}$$
Is this correct?
trigonometry
$endgroup$
add a comment |
$begingroup$
$$begin{align}
(sin x)^4+(cos x)^4
&= (1-cos2x)^2/4 + (1+cos2x)^2/4\
&= (1-2cos2x+(cos2x)^2+1+2cos2x+(cos2x)^2)/4\
&= (1+(cos2x)^2)/2\
end{align}$$
Is this correct?
trigonometry
$endgroup$
add a comment |
$begingroup$
$$begin{align}
(sin x)^4+(cos x)^4
&= (1-cos2x)^2/4 + (1+cos2x)^2/4\
&= (1-2cos2x+(cos2x)^2+1+2cos2x+(cos2x)^2)/4\
&= (1+(cos2x)^2)/2\
end{align}$$
Is this correct?
trigonometry
$endgroup$
$$begin{align}
(sin x)^4+(cos x)^4
&= (1-cos2x)^2/4 + (1+cos2x)^2/4\
&= (1-2cos2x+(cos2x)^2+1+2cos2x+(cos2x)^2)/4\
&= (1+(cos2x)^2)/2\
end{align}$$
Is this correct?
trigonometry
trigonometry
edited Dec 26 '18 at 10:48
Andes Lam
asked Dec 26 '18 at 10:30
Andes LamAndes Lam
213
213
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
A faster way:
begin{align}
sin^4x+cos^4x&=(sin^2x+cos^2x)^2-2sin^2xcos^2x=1-2sin^2xcos^2x\
&=1-frac12(2sin xcos x)^2=1-frac12sin^22x \[1ex]
&=1-frac12frac{1-cos 4x}2=frac{3+cos 4x}4.
end{align}
$endgroup$
$begingroup$
That's exactly the answer I found in other posts. I just wonder why the double angle formula couldn't arrive at the same conclusion at yours...
$endgroup$
– Andes Lam
Dec 26 '18 at 10:45
1
$begingroup$
That's because you didn't go till the end of the linearisation process($cos^22x=frac{1+cos4x}2$).
$endgroup$
– Bernard
Dec 26 '18 at 10:52
add a comment |
$begingroup$
$$cos2y=cos^2y-sin^2y=1-2sin^2y=2cos^2y-1$$
$$impliessin^2y= dfrac{1-cos2y}2 ,cos^2y=dfrac{1+cos2y}2$$
First $y=x$
Apply the same $(y=2x)$ for $cos^22x=dfrac{1+cos4x}2$
$endgroup$
$begingroup$
An error: $$cos^2(y) = frac{1 + cos(2y)}{2}$$ What you have suggests $sin^2y = cos^2y$, which is patently false. Neglecting, of course, that you're not at all addressing OP's question. Where does $x$ even come from in your answer? It's impossible to follow your logic.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:43
$begingroup$
@EeveeTrainer, That was a typo. I introduced to $y$ to replace it with $x,2x$ one by one
$endgroup$
– lab bhattacharjee
Dec 26 '18 at 10:47
$begingroup$
Okay, now I see where you're coming from. Though personally I would elaborate a little more in your answer to demonstrate the utility of this myself. A little elaboration and explanation can go a long way in terms of an answer's usefulness, because if it's impenetrable for other MSE users (IIRC I and someone else had like issues with another of your answers tonight) it's probably not very helpful to a question's OP in case of questions like these. Just my two cents.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:51
$begingroup$
thanks lab, I get it and I make it thru now.
$endgroup$
– Andes Lam
Dec 26 '18 at 10:53
add a comment |
$begingroup$
$$
left(frac{e^{ix}-e^{-ix}}{2i}right)^4+left(frac{e^{ix}+e^{-ix}}{2}right)^4 = frac{1}{2^4}left(left(e^{ix}-e^{-ix}right)^4+left(e^{ix}+e^{-ix}right)^4right)
$$
but
$$
(a+b)^4+(a-b)^4 = 2 (a^4+6 a^2 b^2+b^4)
$$
hence
$$
sin^4 x+cos^4 x = frac{1}{2^3}left(2cos(4x)+6right) = frac{1}{4}left(cos(4x)+3right)
$$
$endgroup$
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
A faster way:
begin{align}
sin^4x+cos^4x&=(sin^2x+cos^2x)^2-2sin^2xcos^2x=1-2sin^2xcos^2x\
&=1-frac12(2sin xcos x)^2=1-frac12sin^22x \[1ex]
&=1-frac12frac{1-cos 4x}2=frac{3+cos 4x}4.
end{align}
$endgroup$
$begingroup$
That's exactly the answer I found in other posts. I just wonder why the double angle formula couldn't arrive at the same conclusion at yours...
$endgroup$
– Andes Lam
Dec 26 '18 at 10:45
1
$begingroup$
That's because you didn't go till the end of the linearisation process($cos^22x=frac{1+cos4x}2$).
$endgroup$
– Bernard
Dec 26 '18 at 10:52
add a comment |
$begingroup$
A faster way:
begin{align}
sin^4x+cos^4x&=(sin^2x+cos^2x)^2-2sin^2xcos^2x=1-2sin^2xcos^2x\
&=1-frac12(2sin xcos x)^2=1-frac12sin^22x \[1ex]
&=1-frac12frac{1-cos 4x}2=frac{3+cos 4x}4.
end{align}
$endgroup$
$begingroup$
That's exactly the answer I found in other posts. I just wonder why the double angle formula couldn't arrive at the same conclusion at yours...
$endgroup$
– Andes Lam
Dec 26 '18 at 10:45
1
$begingroup$
That's because you didn't go till the end of the linearisation process($cos^22x=frac{1+cos4x}2$).
$endgroup$
– Bernard
Dec 26 '18 at 10:52
add a comment |
$begingroup$
A faster way:
begin{align}
sin^4x+cos^4x&=(sin^2x+cos^2x)^2-2sin^2xcos^2x=1-2sin^2xcos^2x\
&=1-frac12(2sin xcos x)^2=1-frac12sin^22x \[1ex]
&=1-frac12frac{1-cos 4x}2=frac{3+cos 4x}4.
end{align}
$endgroup$
A faster way:
begin{align}
sin^4x+cos^4x&=(sin^2x+cos^2x)^2-2sin^2xcos^2x=1-2sin^2xcos^2x\
&=1-frac12(2sin xcos x)^2=1-frac12sin^22x \[1ex]
&=1-frac12frac{1-cos 4x}2=frac{3+cos 4x}4.
end{align}
answered Dec 26 '18 at 10:40
BernardBernard
123k741116
123k741116
$begingroup$
That's exactly the answer I found in other posts. I just wonder why the double angle formula couldn't arrive at the same conclusion at yours...
$endgroup$
– Andes Lam
Dec 26 '18 at 10:45
1
$begingroup$
That's because you didn't go till the end of the linearisation process($cos^22x=frac{1+cos4x}2$).
$endgroup$
– Bernard
Dec 26 '18 at 10:52
add a comment |
$begingroup$
That's exactly the answer I found in other posts. I just wonder why the double angle formula couldn't arrive at the same conclusion at yours...
$endgroup$
– Andes Lam
Dec 26 '18 at 10:45
1
$begingroup$
That's because you didn't go till the end of the linearisation process($cos^22x=frac{1+cos4x}2$).
$endgroup$
– Bernard
Dec 26 '18 at 10:52
$begingroup$
That's exactly the answer I found in other posts. I just wonder why the double angle formula couldn't arrive at the same conclusion at yours...
$endgroup$
– Andes Lam
Dec 26 '18 at 10:45
$begingroup$
That's exactly the answer I found in other posts. I just wonder why the double angle formula couldn't arrive at the same conclusion at yours...
$endgroup$
– Andes Lam
Dec 26 '18 at 10:45
1
1
$begingroup$
That's because you didn't go till the end of the linearisation process($cos^22x=frac{1+cos4x}2$).
$endgroup$
– Bernard
Dec 26 '18 at 10:52
$begingroup$
That's because you didn't go till the end of the linearisation process($cos^22x=frac{1+cos4x}2$).
$endgroup$
– Bernard
Dec 26 '18 at 10:52
add a comment |
$begingroup$
$$cos2y=cos^2y-sin^2y=1-2sin^2y=2cos^2y-1$$
$$impliessin^2y= dfrac{1-cos2y}2 ,cos^2y=dfrac{1+cos2y}2$$
First $y=x$
Apply the same $(y=2x)$ for $cos^22x=dfrac{1+cos4x}2$
$endgroup$
$begingroup$
An error: $$cos^2(y) = frac{1 + cos(2y)}{2}$$ What you have suggests $sin^2y = cos^2y$, which is patently false. Neglecting, of course, that you're not at all addressing OP's question. Where does $x$ even come from in your answer? It's impossible to follow your logic.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:43
$begingroup$
@EeveeTrainer, That was a typo. I introduced to $y$ to replace it with $x,2x$ one by one
$endgroup$
– lab bhattacharjee
Dec 26 '18 at 10:47
$begingroup$
Okay, now I see where you're coming from. Though personally I would elaborate a little more in your answer to demonstrate the utility of this myself. A little elaboration and explanation can go a long way in terms of an answer's usefulness, because if it's impenetrable for other MSE users (IIRC I and someone else had like issues with another of your answers tonight) it's probably not very helpful to a question's OP in case of questions like these. Just my two cents.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:51
$begingroup$
thanks lab, I get it and I make it thru now.
$endgroup$
– Andes Lam
Dec 26 '18 at 10:53
add a comment |
$begingroup$
$$cos2y=cos^2y-sin^2y=1-2sin^2y=2cos^2y-1$$
$$impliessin^2y= dfrac{1-cos2y}2 ,cos^2y=dfrac{1+cos2y}2$$
First $y=x$
Apply the same $(y=2x)$ for $cos^22x=dfrac{1+cos4x}2$
$endgroup$
$begingroup$
An error: $$cos^2(y) = frac{1 + cos(2y)}{2}$$ What you have suggests $sin^2y = cos^2y$, which is patently false. Neglecting, of course, that you're not at all addressing OP's question. Where does $x$ even come from in your answer? It's impossible to follow your logic.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:43
$begingroup$
@EeveeTrainer, That was a typo. I introduced to $y$ to replace it with $x,2x$ one by one
$endgroup$
– lab bhattacharjee
Dec 26 '18 at 10:47
$begingroup$
Okay, now I see where you're coming from. Though personally I would elaborate a little more in your answer to demonstrate the utility of this myself. A little elaboration and explanation can go a long way in terms of an answer's usefulness, because if it's impenetrable for other MSE users (IIRC I and someone else had like issues with another of your answers tonight) it's probably not very helpful to a question's OP in case of questions like these. Just my two cents.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:51
$begingroup$
thanks lab, I get it and I make it thru now.
$endgroup$
– Andes Lam
Dec 26 '18 at 10:53
add a comment |
$begingroup$
$$cos2y=cos^2y-sin^2y=1-2sin^2y=2cos^2y-1$$
$$impliessin^2y= dfrac{1-cos2y}2 ,cos^2y=dfrac{1+cos2y}2$$
First $y=x$
Apply the same $(y=2x)$ for $cos^22x=dfrac{1+cos4x}2$
$endgroup$
$$cos2y=cos^2y-sin^2y=1-2sin^2y=2cos^2y-1$$
$$impliessin^2y= dfrac{1-cos2y}2 ,cos^2y=dfrac{1+cos2y}2$$
First $y=x$
Apply the same $(y=2x)$ for $cos^22x=dfrac{1+cos4x}2$
edited Dec 26 '18 at 10:46
answered Dec 26 '18 at 10:32
lab bhattacharjeelab bhattacharjee
227k15158275
227k15158275
$begingroup$
An error: $$cos^2(y) = frac{1 + cos(2y)}{2}$$ What you have suggests $sin^2y = cos^2y$, which is patently false. Neglecting, of course, that you're not at all addressing OP's question. Where does $x$ even come from in your answer? It's impossible to follow your logic.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:43
$begingroup$
@EeveeTrainer, That was a typo. I introduced to $y$ to replace it with $x,2x$ one by one
$endgroup$
– lab bhattacharjee
Dec 26 '18 at 10:47
$begingroup$
Okay, now I see where you're coming from. Though personally I would elaborate a little more in your answer to demonstrate the utility of this myself. A little elaboration and explanation can go a long way in terms of an answer's usefulness, because if it's impenetrable for other MSE users (IIRC I and someone else had like issues with another of your answers tonight) it's probably not very helpful to a question's OP in case of questions like these. Just my two cents.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:51
$begingroup$
thanks lab, I get it and I make it thru now.
$endgroup$
– Andes Lam
Dec 26 '18 at 10:53
add a comment |
$begingroup$
An error: $$cos^2(y) = frac{1 + cos(2y)}{2}$$ What you have suggests $sin^2y = cos^2y$, which is patently false. Neglecting, of course, that you're not at all addressing OP's question. Where does $x$ even come from in your answer? It's impossible to follow your logic.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:43
$begingroup$
@EeveeTrainer, That was a typo. I introduced to $y$ to replace it with $x,2x$ one by one
$endgroup$
– lab bhattacharjee
Dec 26 '18 at 10:47
$begingroup$
Okay, now I see where you're coming from. Though personally I would elaborate a little more in your answer to demonstrate the utility of this myself. A little elaboration and explanation can go a long way in terms of an answer's usefulness, because if it's impenetrable for other MSE users (IIRC I and someone else had like issues with another of your answers tonight) it's probably not very helpful to a question's OP in case of questions like these. Just my two cents.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:51
$begingroup$
thanks lab, I get it and I make it thru now.
$endgroup$
– Andes Lam
Dec 26 '18 at 10:53
$begingroup$
An error: $$cos^2(y) = frac{1 + cos(2y)}{2}$$ What you have suggests $sin^2y = cos^2y$, which is patently false. Neglecting, of course, that you're not at all addressing OP's question. Where does $x$ even come from in your answer? It's impossible to follow your logic.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:43
$begingroup$
An error: $$cos^2(y) = frac{1 + cos(2y)}{2}$$ What you have suggests $sin^2y = cos^2y$, which is patently false. Neglecting, of course, that you're not at all addressing OP's question. Where does $x$ even come from in your answer? It's impossible to follow your logic.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:43
$begingroup$
@EeveeTrainer, That was a typo. I introduced to $y$ to replace it with $x,2x$ one by one
$endgroup$
– lab bhattacharjee
Dec 26 '18 at 10:47
$begingroup$
@EeveeTrainer, That was a typo. I introduced to $y$ to replace it with $x,2x$ one by one
$endgroup$
– lab bhattacharjee
Dec 26 '18 at 10:47
$begingroup$
Okay, now I see where you're coming from. Though personally I would elaborate a little more in your answer to demonstrate the utility of this myself. A little elaboration and explanation can go a long way in terms of an answer's usefulness, because if it's impenetrable for other MSE users (IIRC I and someone else had like issues with another of your answers tonight) it's probably not very helpful to a question's OP in case of questions like these. Just my two cents.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:51
$begingroup$
Okay, now I see where you're coming from. Though personally I would elaborate a little more in your answer to demonstrate the utility of this myself. A little elaboration and explanation can go a long way in terms of an answer's usefulness, because if it's impenetrable for other MSE users (IIRC I and someone else had like issues with another of your answers tonight) it's probably not very helpful to a question's OP in case of questions like these. Just my two cents.
$endgroup$
– Eevee Trainer
Dec 26 '18 at 10:51
$begingroup$
thanks lab, I get it and I make it thru now.
$endgroup$
– Andes Lam
Dec 26 '18 at 10:53
$begingroup$
thanks lab, I get it and I make it thru now.
$endgroup$
– Andes Lam
Dec 26 '18 at 10:53
add a comment |
$begingroup$
$$
left(frac{e^{ix}-e^{-ix}}{2i}right)^4+left(frac{e^{ix}+e^{-ix}}{2}right)^4 = frac{1}{2^4}left(left(e^{ix}-e^{-ix}right)^4+left(e^{ix}+e^{-ix}right)^4right)
$$
but
$$
(a+b)^4+(a-b)^4 = 2 (a^4+6 a^2 b^2+b^4)
$$
hence
$$
sin^4 x+cos^4 x = frac{1}{2^3}left(2cos(4x)+6right) = frac{1}{4}left(cos(4x)+3right)
$$
$endgroup$
add a comment |
$begingroup$
$$
left(frac{e^{ix}-e^{-ix}}{2i}right)^4+left(frac{e^{ix}+e^{-ix}}{2}right)^4 = frac{1}{2^4}left(left(e^{ix}-e^{-ix}right)^4+left(e^{ix}+e^{-ix}right)^4right)
$$
but
$$
(a+b)^4+(a-b)^4 = 2 (a^4+6 a^2 b^2+b^4)
$$
hence
$$
sin^4 x+cos^4 x = frac{1}{2^3}left(2cos(4x)+6right) = frac{1}{4}left(cos(4x)+3right)
$$
$endgroup$
add a comment |
$begingroup$
$$
left(frac{e^{ix}-e^{-ix}}{2i}right)^4+left(frac{e^{ix}+e^{-ix}}{2}right)^4 = frac{1}{2^4}left(left(e^{ix}-e^{-ix}right)^4+left(e^{ix}+e^{-ix}right)^4right)
$$
but
$$
(a+b)^4+(a-b)^4 = 2 (a^4+6 a^2 b^2+b^4)
$$
hence
$$
sin^4 x+cos^4 x = frac{1}{2^3}left(2cos(4x)+6right) = frac{1}{4}left(cos(4x)+3right)
$$
$endgroup$
$$
left(frac{e^{ix}-e^{-ix}}{2i}right)^4+left(frac{e^{ix}+e^{-ix}}{2}right)^4 = frac{1}{2^4}left(left(e^{ix}-e^{-ix}right)^4+left(e^{ix}+e^{-ix}right)^4right)
$$
but
$$
(a+b)^4+(a-b)^4 = 2 (a^4+6 a^2 b^2+b^4)
$$
hence
$$
sin^4 x+cos^4 x = frac{1}{2^3}left(2cos(4x)+6right) = frac{1}{4}left(cos(4x)+3right)
$$
answered Dec 26 '18 at 11:56
CesareoCesareo
9,3613517
9,3613517
add a comment |
add a comment |
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