# Thread: Integration by trigonometric substitution

1. ## Integration by trigonometric substitution

Could someone check whether I've got the correct answer for the following integrals?

1. $\displaystyle\int{\dfrac{dx}{x^2\sqrt{x^2+16}}$

I used the trigonometric substitution $x=4\tan\theta$ and got this as my answer

$dx=4\sec^2\theta \ d\theta$

$\Rightarrow\displaystyle\int{\dfrac{dx}{x^2\sqrt{x ^2+16}}=\int{\frac{4\sec^2\theta}{16\tan^2\theta\s qrt{16\tan^2\theta+16}}} \ d\theta$

$=\displaystyle\int{\frac{4\sec^2\theta}{16\tan^2\t heta\cdot 4\sec\theta}} \ d\theta$

$=\displaystyle\int{\frac{\sec\theta}{16\tan^2\thet a}} \ d\theta$

$=\displaystyle\frac{1}{16}\int{\frac{\cos\theta}{\ sin^2\theta}} \ d\theta$

Let $u=\sin\theta\Rightarrow du=\cos\theta \ d\theta$

$=\displaystyle\frac{1}{16}\int{\frac{1}{u^2}} \ du$

$=\displaystyle -\frac{1}{16u}+C$

$-\dfrac{1}{16\sin\theta} + C$

$\tan\theta=\dfrac{x}{4}\Rightarrow \sin\theta=\dfrac{x}{\sqrt{x^2+16}}$

$=-\dfrac{\sqrt{x^2+16}}{16x}+C$