Question:
Evaluate the following integrals:
$\int \frac{x \cos ^{-1} x}{\sqrt{1-x^{2}}} d x$
Solution:
Let $I=\int \frac{x \cos ^{-1} x}{\sqrt{1-x^{2}}} d x$
Let $\mathrm{t}=\cos ^{-1} \mathrm{x}$
$\mathrm{dt}=\frac{1}{\sqrt{1-\mathrm{x}^{2}}} \mathrm{dx}$
Also,
$\cos t=x$
Thus,
$I=-\int t \cos t d t$
Now let us solve this by 'by parts' method
Using integration by parts,
$I=-t\left(\int \cos t d t-\int \frac{d}{d t} t \int \cos t d t\right)$
Let
$\mathrm{U}=\mathrm{t} ; \mathrm{d} \mathrm{u}=\mathrm{dt}$
$\int \cos t \mathrm{dt}=\mathrm{v} ; \sin \mathrm{t}=\mathrm{dv}$
Thus,
$I=-\left[t \sin t-\int \sin t d t\right]$
$I=-[t \sin t+\cos t]+c$
Substituting
$t=\cos ^{-1} x$
$I=-\left[\cos ^{-1} x \sin t+x\right]+c$
$I=-\left[\cos ^{-1} X \sqrt{1-X^{2}}+X\right]+c$