Evaluación de $\int\frac{\sqrt{\cos 2x}}{\sin x}\,dx$

Question

Calcular la integral indefinida $$ \int\frac{\sqrt{\cos 2x}}{\sin x}\,dx $$

Mi intento:

$$ \begin{align} \int\frac{\sqrt{\cos 2x}}{\sin x}\,dx &= \int\frac{\cos 2x}{\sin^2 x\sqrt{\cos 2x}}\sin xdx\\ &= \int\frac{2\cos^2 x-1}{(1-\cos^2 x)\sqrt{2\cos^2 x-1} }\sin x \,dx \end{align} $$

Dejemos que $\cos x = t$ para que $\sin x\,dx = -dt$ . Esto cambia la integral a

$$ \begin{align} \int\frac{(2t^2-1)}{(t^2-1)\sqrt{2t^2-1}}\,dt &= \int\frac{(2t^2-2)+1}{(t^2-1)\sqrt{2t^2-1}}\,dt\\ &= 2\int\frac{dt}{\sqrt{2t^2-1}}+\int \frac{dt}{(t^2-1)\sqrt{2t^2-1}} \end{align} $$

¿Cómo puedo resolver la integral desde aquí?

Answer 1

\begin{align} \int\frac{\sqrt{\cos 2x}}{\sin x}\ dx&=\int\frac{\sqrt{\cos^2x-\sin^2x}}{\sin x}\ dx\\ &\stackrel{\color{red}{[1]}}=\int\frac{\sqrt{t^4-6t^2+1}}{t^3+t}\ dt\\ &\stackrel{\color{red}{[2]}}=\frac12\int\frac{\sqrt{u^2-6u+1}}{u^2+u}\ du\\ &\stackrel{\color{red}{[3]}}=\int\frac{(y^2-6y+1)^2}{(y-1)(y-3)(y+1)(y^2+2t-7)}\ dy\\ &\stackrel{\color{red}{[4]}}=\int\left[\frac1{y-1}+\frac1{y-3}-\frac1{y+1}-\frac{16}{y^2+2y-7}\right]\ dt\\ &=\int\left[\frac1{y-1}+\frac1{y-3}-\frac1{y+1}-\frac{16}{(y+1)^2-8}\right]\ dt \end{align} El resto es tuyo.

---

Notas :

$\color{red}{[1]}\;\;\;$ Utilice Sustitución de Weierstrass , $\tan\left(\dfrac{x}{2}\right)=t$ .

$\color{red}{[2]}\;\;\;$ Utilizar la sustitución $u=t^2$ .

$\color{red}{[3]}\;\;\;$ Utilice Sustitución de Euler , $y-u=\sqrt{u^2-6u+1}\;\color{blue}{\Rightarrow}\;y=\dfrac{u^2-1}{2u-6}$ .

$\color{red}{[4]}\;\;\;$ Utilice descomposición de fracciones parciales .

Answer 2

Si en la última integral sustituyes $t=\frac{1}{\sqrt{2}}\cosh z$ terminas con..: $$ I = \frac{1}{\sqrt{2}}\int\frac{1}{\frac{\cosh^2 z}{2}-1}dz=-\operatorname{arctanh}(\sqrt{2}\tanh z)=-\operatorname{arctanh}\left(\sqrt{2-\frac{1}{t^2}}\right).$$

Answer 3

$\displaystyle I=\int\frac{\sqrt{2t^2-1}}{t^2-1}dt$ Así que $\displaystyle t=\frac{1}{\sqrt{2}}\sec\theta, dt=\frac{1}{\sqrt{2}}\sec\theta\tan\theta d\theta$ da

$\displaystyle I=\frac{1}{\sqrt{2}}\int\frac{\sec\theta\tan^2\theta}{\frac{1}{2}\sec^2\theta-1}d\theta=\sqrt{2}\int\frac{\sec\theta\tan^2\theta}{\sec^2\theta-2}d\theta=\sqrt{2}\int\frac{\sin^2\theta\cos\theta}{\cos^2\theta(1-2\cos^2\theta)}d\theta$ .

Próximo alquiler $u=\sin\theta, du=\cos\theta d\theta$ da

$\displaystyle I=\sqrt{2}\int\frac{u^2}{(1-u^2)(2u^2-1)}du=\sqrt{2}\cdot\frac{1}{2}\int\left(\frac{1}{1-u}+\frac{1}{1+u}-\frac{1}{\sqrt{2}u+1}+\frac{1}{\sqrt{2}u-1}\right)du$

$\displaystyle=\frac{1}{\sqrt{2}}\left[\ln\left(\frac{1+u}{1-u}\right)+\frac{1}{\sqrt{2}}\ln\left(\frac{\sqrt{2}u-1}{\sqrt{2}u+1}\right)\right]+C$

$\displaystyle=\frac{1}{\sqrt{2}}\left[\ln\left(\frac{1+\sin\theta}{1-\sin\theta}\right)+\frac{1}{\sqrt{2}}\ln\left(\frac{\sqrt{2}\sin\theta-1}{\sqrt{2}\sin\theta+1}\right)\right]+C=\frac{1}{\sqrt{2}}\left[2\ln\lvert\sec\theta+\tan\theta\rvert+\frac{1}{\sqrt{2}}\ln\left(\frac{\sqrt{2}\sin\theta-1}{\sqrt{2}\sin\theta+1}\right)\right]+C$

$\displaystyle=\sqrt{2}\ln\big|\sqrt{2}t+\sqrt{2t^2-1}\big|+\frac{1}{2}\ln\bigg|\frac{\sqrt{2t^2-1}-t}{\sqrt{2t^2-1}+t}\bigg|+C$

$=\displaystyle\sqrt{2}\ln\bigg|\sqrt{2}\cos x+\sqrt{\cos 2x}\bigg|+\ln\bigg|\frac{\cos x-\sqrt{\cos 2x}}{\sin x}\bigg|+C$

Answer 4

Poner $$\cos 2x =\frac{1 -\tan^2x}{1+\tan^2x}$$ $$\int\frac{\sqrt{1-\tan^2 x}}{\tan x}$$ $$1-\tan^2 x =t^2$$ $$\implies -2\tan x \sec^2 x dx=2tdt$$ $$\int\frac{t}{\tan x}.\frac{-tdt}{\tan x\sec^2 x}=-\int\frac{t^2}{(1-t^2)(2-t^2)}$$ $$=-\int\left[\frac{1}{(1-t^2)} -\frac{2}{(2-t^2)}\right]dt$$ $$=-\frac{1}{2}\log\left|{\frac{1+t}{1-t}}\right| +\frac{1}{\sqrt2}\log\left|{\frac{\sqrt2+t}{\sqrt2-t}}\right|$$

Answer 5

La magia aparece al realizar el cambio $u=\dfrac{\cos(x)}{\sqrt{\cos(2x)}}$ debido al hecho de que hay $\sin(x)$ en el denominador.

La integral pasa a ser $I=\displaystyle\int\dfrac {\mathop{du}}{(u^2-1)(2u^2-1)}\quad$ con la raíz cuadrada no más.

Ahora es sólo $\dfrac 1{u^2-1}-\dfrac{2}{2u^2-1}$

Y $$I=\sqrt{2}\operatorname{argth}(\sqrt{2}\,u)-\operatorname{argth}(u)$$

Nota: $\operatorname{argth}=\tanh^{-1}$