# Help evaluating integral

• May 12th 2011, 01:24 AM
dnftp
Help evaluating integral
Could someone please help evaluate the integral $\displaystyle \int {x}^{5}\sqrt{1-{x}^{3} }$
• May 12th 2011, 01:59 AM
Glitch
Quote:

Originally Posted by dnftp
Could someone please help evaluate the integral $\displaystyle \int {x}^{5}\sqrt{1-{x}^{3} }$

Let $\displaystyle u = 1 - x^3$

$\displaystyle du = - 3x^2 dx$

So we have,
$\displaystyle -\frac{1}{3}\int {x}^{3}\sqrt{u} \ du$

But we said $\displaystyle u = 1 - x^3$, so rearranging we get $\displaystyle x^3 = (1 - u)$

So sub that in:

= $\displaystyle -\frac{1}{3}\int(1 - u)\sqrt{u} \ du$

= $\displaystyle -\frac{1}{3}\int\sqrt{u} - u^{3/2} \ du$

Now it's easy. Just remember to sub in the value of u after you've done the integration.
• May 12th 2011, 02:23 AM
FernandoRevilla
Alternatively, we can consider it as a binomial differential

$\displaystyle I = \int x^m(a+b x^n)^p\;dx\quad (m,n,p\in\mathbb{Q})$

In our case

$\displaystyle \dfrac{m+1}{n}\in\mathbb{Z}$

So, with the substitution

$\displaystyle a+bx^n=t^s\;,\quad s=\textrm{denom}(p)$

we obtain a rational integral on t.