Evaluation of $\int\frac{\sqrt{\cos 2x}}{\sin x}\,dx$

Question

Compute the indefinite integral $$ \int\frac{\sqrt{\cos 2x}}{\sin x}\,dx $$

My Attempt:

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

Let $\cos x = t$, so that $\sin x\,dx = -dt$. This changes the integral to

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

How can I solve the integral from here?

The substitution $\cos x = t$ is only allowed in a small interval, since $\cos$ must be injective on the interval of integration... — AlexR, Jul 23 '14 at 18:42
The integral won't make much sense, either, on anything but a small interval. @AlexR — Semiclassical, Jul 23 '14 at 18:47
@Semiclassical jep, it must be a subset of $(0,\pi) + k\pi$ for some $k\in\mathbb Z$ for the integrand to be non-singular. And on this interval the substitution is all right :) — AlexR, Jul 23 '14 at 18:51
https://math.stackexchange.com/questions/887082/integral-of-frac-sqrt-cos-2-x-sin-x — user236182, Sep 10 '17 at 19:14
See also: Integrating $\int\frac{\sqrt{\cos 2x}}{\sin x},\text{d}x$. — Martin Sleziak, Jan 22 '19 at 10:51

score 16 · Accepted Answer · answered Jul 23 '14 at 19:51

\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} The rest is yours.

Notes :

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

$\color{red}{[2]}\;\;\;$Use substitution $u=t^2$.

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

$\color{red}{[4]}\;\;\;$Use partial fractions decomposition.

You mean $u=\frac{y^2-1}{2y-6}$. Read more about Euler substitution. — user236182, Sep 10 '17 at 19:00

score 5 · Answer 2 · answered Jul 23 '14 at 19:25

5

If in the last integral you substitute $t=\frac{1}{\sqrt{2}}\cosh z$, you end with: $$ I = \frac{1}{\sqrt{2}}\int\frac{1}{\frac{\cosh^2 z}{2}-1}dz=-\operatorname{arctanh}(\sqrt{2}\tanh z)=-\operatorname{arctanh}\left(\sqrt{2-\frac{1}{t^2}}\right).$$

answered Jul 23 '14 at 19:25

Jack D'Aurizio

361,689

score 4 · Answer 3 · answered Jul 15 '15 at 01:32

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

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

Next letting $u=\sin\theta, du=\cos\theta d\theta$ gives

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

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

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

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

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

Good solution very nice – Ekaveera Gouribhatla Aug 10 '18 at 15:57 — Ekaveera Gouribhatla, Aug 10 '18 at 15:57

Aakash Kumar · Answer 4 · 2016-08-10T17:06:24.443

4

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

edited Aug 10 '16 at 17:06

answered Aug 10 '16 at 17:00

Aakash Kumar

3,576

score 1 · Answer 5 · answered Jan 22 '19 at 14:56

The magic appears when performing the change $u=\dfrac{\cos(x)}{\sqrt{\cos(2x)}}$ due to the fact that there is $\sin(x)$ on denominator.

The integral is changed to $I=\displaystyle\int\dfrac {\mathop{du}}{(u^2-1)(2u^2-1)}\quad$ with not square root anymore.

Now it is just $\dfrac 1{u^2-1}-\dfrac{2}{2u^2-1}$

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

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

Evaluation of $\int\frac{\sqrt{\cos 2x}}{\sin x}\,dx$

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