# Evaluating a integral that looks tough

Can anybody help me find the value of this integral? I utterly have no clue on how to solve this and $$r$$ is a constant.

$$\int \frac{dx}{\left({x^2+r^2}\right)^{3/2}}$$

The answer is given as $$\dfrac{x}{{\left(x^2+r^2\right)}^{1/2}}$$

I even tried differentiating the answer, but didn't get the original function back.

It's simpler if you first get rid of $$r$$: substitute $$x=rt$$, so your integral becomes $$\int\frac{1}{r^3(t^2+1)^{3/2}}r\,dt$$ We can ignore $$1/r^2$$ for the time being and reinsert it at the end: now $$\int\frac{1}{(t^2+1)^{3/2}}\,dt= \int\frac{t^2+1-t^2}{(t^2+1)^{3/2}}\,dt= \int\frac{1}{(t^2+1)^{1/2}}\,dt+ \int t\frac{-t}{(t^2+1)^{3/2}}\,dt$$ Do the last by parts as indicated: $$\int t\frac{-t}{(t^2+1)^{3/2}}\,dt= t\frac{1}{(t^2+1)^{1/2}}-\int\frac{1}{(t^2+1)^{1/2}}\,dt$$ The two integrals now cancel! $$\int\frac{1}{(t^2+1)^{3/2}}\,dt=\frac{t}{(t^2+1)^{1/2}}+c$$ Reinsert the factor $$1/r^2$$ and back substitute: $$\int\frac{1}{(x^2+r^2)^{3/2}}\,dx= \frac{1}{r^2}\frac{x/r}{(x^2/r^2+1)^{1/2}}+c=\frac{1}{r^2}\frac{x}{(x^2+r^2)^{1/2}}+c$$

• Of all the answers this is the most amazing and creative. I love it ! Mar 24, 2019 at 13:00
• @Maxwell'sGhost Wicked trick, isn't it? Mar 24, 2019 at 13:29
• yeah ! pretty wicked . Doing things by part and cancelling out weird integrals is a skill that i have always wanted to acquire Mar 24, 2019 at 13:32

I get \begin{align} \frac d{dx}\frac x{(x^2+r^2)^{1/2}} &=\frac1{(x^2+r^2)^{1/2}}-\frac{x^2}{(x^2+r^2)^{3/2}}\\ &=\frac{x^2+r^2-x^2}{(x^2+r^2)^{3/2}}=\frac{r^2}{(x^2+r^2)^{3/2}} \end{align} which is your original integrand, save for a constant factor $$r^2$$.

Systematic ways to attack this include the substitutions $$x=r\tan t$$ or $$x=r\sinh y$$.

• thank you for showing me by differentiating back. Heck i was so dumb, after seeing your answer i understood my mistake instantly Mar 24, 2019 at 12:59

You can compute that integral doing the substitution $$x=r\tan\theta$$ and $$\mathrm dx=r(1+\tan^2\theta)\,\mathrm d\theta$$. The correct answer is $$\dfrac x{r^2(x^2+r^2)^{\frac12}}$$.

Set $$x=r\tan\theta\implies \mathrm dx=r\sec^2\theta\mathrm d\theta$$. \begin{aligned}\int\dfrac{1}{(x^2+r^2)^{3/2}}\mathrm dx&\mapsto \int\dfrac{r\sec^2\theta}{r^3\sec^3\theta}\mathrm d\theta\\&=\dfrac{1}{r^2}\int\cos\theta\mathrm d\theta\\&=\dfrac{1}{r^2}\sin\theta=\dfrac{1}{r^2}\cdot\dfrac{x}{\sqrt{x^2+r^2}}+C\end{aligned}