integración de funciones trigonométricas

Question

Tengo una pregunta sobre las siguientes dos integrales;

$\int_0^{2\pi} \frac{\sin\theta}{A-\sin\theta} d\theta$ y $\int_0^{2\pi} \frac{1}{A-\sin\theta} d\theta$ donde $A>0$ . Qué método se puede utilizar para integrarlos.

Answer 1

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$\ds{\int_{0}^{2\pi}{\sin\pars{\theta} \over A - \sin\pars{\theta}}\,\dd \theta = -2\pi + A\int_{0}^{2\pi}{\dd\theta \over A - \sin\pars{\theta}}}$ . Voy a asumir $\ds{A \in \mathbb{R}\setminus\bracks{-1,1}}$ .

$$\begin{align} \int_{0}^{2\pi}{\dd\theta \over A - \sin\pars{\theta}} & = \int_{-\pi}^{\pi}{\dd\theta \over A + \sin\pars{\theta}} = \int_{0}^{\pi}\bracks{{1 \over A + \sin\pars{\theta}} + {1 \over A - \sin\pars{\theta}}}\dd\theta \\[5mm] & = 2A\int_{0}^{\pi}{\dd\theta \over A^{2} - \sin^{2}\pars{\theta}} = 2A\int_{-\pi/2}^{\pi/2}{\dd\theta \over A^{2} - \cos^{2}\pars{\theta}} = 4A\int_{0}^{\pi/2}{\dd\theta \over A^{2} - \cos^{2}\pars{\theta}} \\[5mm] & = 4A\int_{0}^{\pi/2}{\sec^{2}\pars{\theta} \over A^{2}\sec^{2}\pars{\theta} - 1} \,\dd\theta = 4A\int_{0}^{\pi/2}{\sec^{2}\pars{\theta} \over A^{2}\tan^{2}\pars{\theta} + A^{2} - 1}\,\dd\theta \\[5mm] & = 4A\,{1 \over A^{2} - 1}\,{\root{A^{2} - 1} \over \verts{A}}\int_{0}^{\pi/2} {\verts{A}\sec^{2}\pars{\theta}/\root{A^{2} - 1} \over \bracks{\verts{A}\tan\pars{\theta}/\root{A^{2} - 1}}^{2} + 1}\,\dd\theta \\[5mm] & = {4\,\mrm{sgn}\pars{A} \over \root{A^{2} - 1}}\ \underbrace{\int_{0}^{\infty}{\dd t \over t^{2} + 1}}_{\ds{=\ {\pi \over 2}}}\ =\ \bbx{2\pi\,{\mrm{sgn}\pars{A} \over \root{A^{2} - 1}}} \end{align}$$

donde $\ds{t = {\verts{A} \over \root{A^{2} - 1}}\,\tan\pars{\theta}}$ .

Answer 2

Sugerencia: sustituirlo por $$\sin(\theta)=\frac{2t}{1+t^2}$$ y $$d\theta=\frac{2}{1+t^2}dt$$ Su nueva integral viene dada por $$\int 4\,{\frac {t}{ \left( A{t}^{2}+A-2\,t \right) \left( {t}^{2}+1 \right) }} dt$$ para integrar el uso que $$\frac{4t}{(At^2-2t+A)(t^2+1)}=4+1/5\,{\frac {1}{ \left( t-2 \right) A}}+1/5\,{\frac {-t-2}{ \left( { t}^{2}+1 \right) A}}$$

Answer 3

Una pista:

Considere la sustitución $$u=\sin \theta$$ y la identidad $$\cos\theta =\pm\sqrt{1-\sin^2\theta}$$