Integral of $\log (1+a\cos x)$ from 0 to $\pi$

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In summary, the problem involves evaluating the integral of the logarithm of a trigonometric function using Leibnitz's Rule. The attempt at a solution involved using a substitution and changing the limits of integration, but this approach may not work due to the sign change in the function. It is safer to split the integral into two parts.
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utkarshakash
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Homework Statement


Evaluate [itex]\displaystyle \int_0^{\pi} \log (1+a\cos x) dx[/itex]

Homework Equations



The Attempt at a Solution


Using Leibnitz's Rule,
F'(a)=[itex]\displaystyle \int_0^{\pi} \dfrac{\cos x}{1+a \cos x} dx [/itex]

Now, If I assume sinx=t, then the above integral changes to
[itex]\displaystyle \int_0^{0} \dfrac{dt}{1+a \sqrt{1-t^2}} [/itex]

Since both the limits are zero now, shouldn't the value of integral be 0! :confused:
 
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utkarshakash said:
[itex]\displaystyle \int_0^{0} \dfrac{dt}{1+a \sqrt{1-t^2}} [/itex]

Since both the limits are zero now, shouldn't the value of integral be 0! :confused:
No. For one thing, the use of the square root function hides the fact that cos(t) will change sign over the range. Split it into two integrals to be safe.
 

1. What is the formula for the integral of $\log (1+a\cos x)$ from 0 to $\pi$?

The formula for the integral of $\log (1+a\cos x)$ from 0 to $\pi$ is $\int_0^\pi \log (1+a\cos x) dx = \pi \log \left(\frac{(1+a)^2}{2a}\right)$.

2. What does the integral of $\log (1+a\cos x)$ from 0 to $\pi$ represent?

The integral of $\log (1+a\cos x)$ from 0 to $\pi$ represents the area under the curve of the function $\log (1+a\cos x)$ from 0 to $\pi$ on the x-axis.

3. How is the integral of $\log (1+a\cos x)$ from 0 to $\pi$ related to trigonometric functions?

The integral of $\log (1+a\cos x)$ from 0 to $\pi$ is related to trigonometric functions through the use of the trigonometric identity $\cos x = \frac{e^{ix}+e^{-ix}}{2}$.

4. What is the significance of the constant "a" in the integral of $\log (1+a\cos x)$ from 0 to $\pi$?

The constant "a" in the integral of $\log (1+a\cos x)$ from 0 to $\pi$ represents the amplitude of the cosine function within the logarithm. It affects the shape and position of the curve and ultimately the value of the integral.

5. Can the integral of $\log (1+a\cos x)$ from 0 to $\pi$ be evaluated using numerical methods?

Yes, the integral of $\log (1+a\cos x)$ from 0 to $\pi$ can be evaluated using numerical methods such as the trapezoidal rule, Simpson's rule, or Gaussian quadrature. These methods approximate the integral by breaking it into smaller segments and using mathematical techniques to calculate the area under each segment.

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