Saturday, September 14, 2019

calculus - Integration by Euler's formula




How do you integrate the following by using Euler's formula, without using integration by parts? $$I=\displaystyle\int \dfrac{3+4\cos {\theta}}{(3\cos {\theta}+4)^2}d\theta$$



I did integrate it by parts, by writing the $3$ in the numerator as $3\sin^2 {\theta}+3\cos^2{\theta}$, and then splitting the numerator.



But can it be solved by using complex numbers and the Euler's formula?


Answer



Hint



When you have an expression with a squared denominator, you could think that the solution is of the form $$I=\displaystyle\int \dfrac{3+4\cos {\theta}}{(3\cos {\theta}+4)^2}~d\theta=\frac{a+b\sin \theta+c\cos \theta}{3\cos {\theta}+4}$$ Differentiate the rhs and identify terms. You will get very simple results.



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