MHB Can the derivative of the given integral be simplified to -A?

AI Thread Summary
The discussion centers around proving the integral identity involving trigonometric functions and logarithms. Participants debate the validity of a proposed identity, highlighting that substituting specific values leads to contradictions. The conversation progresses to the differentiation of the integral using Leibniz's rule, with various substitutions and transformations suggested for simplification. Ultimately, one participant claims to have derived that the derivative of the integral equals -A, prompting a request for verification of their calculations. The conversation emphasizes the complexity of proving the original integral identity.
Suvadip
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Can it be proved?
$$\left(\frac{-2\sin A}{1-\cos A}\right)\cos\left(\frac{A}{2}\right)\tan^{-1}\left[\cos \left(\frac{A}{2}\right)\right]=\frac{\pi^2-4A^2}{8}$$
 
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suvadip said:
Can it be proved?
$$\left(\frac{-2\sin A}{1-\cos A}\right)\cos\left(\frac{A}{2}\right)\tan^{-1}\left[\cos \left(\frac{A}{2}\right)\right]=\frac{\pi^2-4A^2}{8}$$

Hi suvadip, :)

This trigonometric identity is not valid. For example substituting \(A=\dfrac{\pi}{2}\) we get zero in the right hand side whereas \(-\dfrac{1}{\sqrt{2}}\tan^{-1}\dfrac{1}{\sqrt{2}}\neq 0\) on the right hand side.
 
Sudharaka said:
Hi suvadip, :)

This trigonometric identity is not valid. For example substituting \(A=\dfrac{\pi}{2}\) we get zero in the right hand side whereas \(-\dfrac{1}{\sqrt{2}}\tan^{-1}\dfrac{1}{\sqrt{2}}\neq 0\) on the right hand side.

Please look at the attached sheet. How to reach the final answer
 

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suvadip said:
Please look at the attached sheet. How to reach the final answer

Your attachment is not very clear and I cannot read it. If you can post a more clear attachment or write down the equations using LaTeX I would perhaps be able to help. :)
 
Sudharaka said:
Your attachment is not very clear and I cannot read it. If you can post a more clear attachment or write down the equations using LaTeX I would perhaps be able to help. :)
I have to prove
$$\int_0^{\pi/2}\frac{log(1+cosA cosx)}{cosx}dx=\frac{\pi^2-4A^2}{8}$$

I have arrived at

$$\frac{d}{dA}(I)=-sinA \int_0^1\frac{2}{1+cosA +(1-cos A)z^2}dz$$ where I is the given integral. Am I correct so far and how to proceed at the answer from there? I have used the Leibnitz's rule for differentiation under the sign of integration.
 
suvadip said:
I have to prove
$$\int_0^{\pi/2}\frac{log(1+cosA cosx)}{cosx}dx=\frac{\pi^2-4A^2}{8}$$

I have arrived at

$$\frac{d}{dA}(I)=-sinA \int_0^1\frac{2}{1+cosA +(1-cos A)z^2}dz$$

How do you get this? After differentiating wrt A, I get,
$$\frac{dI}{dA}=\int_0^{\pi/2}\frac{-\sin A}{1+\cos A \cos x}dx$$
Can you integrate $\frac{dx}{1+a\cos x}$? Hint: Use $\cos x=\frac{1-\tan^2(x/2)}{1+\tan^2(x/2)}$.

EDIT: I see that you already tried that. Try this, it makes the algebra a bit easier to handle.
$$\frac{dI}{dA}=\int_0^{\pi/2}\frac{-\sin A}{1+\cos A \cos x}dx=\int_0^{\pi/2}\frac{-\sin A}{1+\cos A \sin x}dx$$
Use $\sin x=\frac{2\tan(x/2)}{1+\tan^2(x/2)}$ to get
$$\frac{dI}{dA}=-\sin A\int_0^{\pi/2} \frac{\sec^2(x/2)}{1+\tan^2(x/2)+2\cos A\tan(x/2)}dx$$
Use the substitution $\tan(x/2)=t$,
$$\Rightarrow \frac{dI}{dA}=-2\sin A\int_0^1 \frac{dt}{t^2+2t\cos A+1}=-2\sin A\int_0^1 \frac{dt}{(t+\cos A)^2+\sin^2A}$$
I suppose you can solve after this. :)
 
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suvadip said:
I have to prove
$$\int_0^{\pi/2}\frac{\log(1+\cos A \cos x)}{\cos x}dx=\frac{\pi^2-4A^2}{8}$$

I have arrived at

$$\frac{d}{dA}(I)=-\sin A \int_0^1\frac{2}{1+\cos A +(1-\cos A)z^2}dz$$ where I is the given integral. Am I correct so far and how to proceed at the answer from there? I have used the Leibnitz's rule for differentiation under the sign of integration.
You are correct so far (having differentiated under the integral sign and then made the substitution $z = \tan(x/2)$). The next step is to write this as $$\frac{d}{dA}(I)=\frac{-2\sin A}{1-\cos A} \int_0^1\frac{1}{z^2 +\frac{1+\cos A}{1-\cos A}}dz.$$ Now use the fact that $$\frac{1+\cos A}{1-\cos A} = \cot^2(A/2)$$ to get $$\frac{d}{dA}(I)= \frac{-2\sin A}{1-\cos A} \int_0^1\frac{1}{z^2 +\cot^2(A/2)}dz = \frac{-2\sin A}{1-\cos A} \Bigl[\tan(A/2)\arctan\bigl(z\tan(A/2)\bigr)\Bigr]_0^1 = \frac{-2\sin A}{1-\cos A}\frac A2\tan(A/2)$$ (provided that $|A| < \pi$, so that $\arctan\bigl(\tan(A/2)\bigr) = A/2$). Using half-angle formulas again, you can write this as $$\frac{\frac{-2}{1+\tan^2(A/2)}}{1- \frac{1-\tan^2(A/2)}{1+\tan^2(A/2)}}A\tan(A/2) = \frac{-A\tan(A/2)}{\tan^2(A/2)} = -A\cot(A/2).$$ If that last expression was just $A$ (without the $\cot(A/2)$) then you could integrate it to get $-A^2/2$ which, together with the initial value $0$ when $A = \pi/2$, would give the formula that you are looking for.

But, unless I have made some silly mistake, that extra $\cot(A/2)$ is there, and that gives a function $-A\cot(A/2)$ that does not have an elementary integral.
 
Hi Opalg! :)

I am not sure but I seem to be getting the final answer. If the integral I reached is evaluated further, I get
$$\frac{dI}{dA}=2\left(\tan^{-1}\left(\frac{\cos A}{\sin A}\right)-\tan^{-1}\left(\frac{1+\cos A}{\sin A}\right)\right)$$
Using the formula for $\tan^{-1}a-\tan^{-1}b$, I simplified it to
$$\frac{dI}{dA}=-A$$
Please check if I did anything wrong, thank you. :)
 
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Pranav said:
Hi Opalg! :)

I am not sure but I seem to be getting the final answer. If the integral I reached is evaluated further, I get
$$\frac{dI}{dA}=2\left(\tan^{-1}\left(\frac{\cos A}{\sin A}\right)-\tan^{-1}\left(\frac{1+\cos A}{\sin A}\right)\right)$$
Using the formula for $\tan^{-1}a-\tan^{-1}b$, I simplified it to
$$\frac{dI}{dA}=-A$$
Please check if I did anything wrong, thank you. :)
I'm very willing to believe that you're right, but I'll leave it to the OP to sort out the details. (Wink)
 
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