# Inegration Frustration

1. Jan 26, 2009

### negatifzeo

1. The problem statement, all variables and given/known data
$\int (9-x^2)^{5/2} dx$

2. Relevant equations

3. The attempt at a solution
Letting$$x=3sin\theta$$ and $$dx=3 cos \theta$$,

[itex] 243\int(1-sin^2\theta)^{5/2}3 cos \theta d\theta[/tex]

Taking the 3 out gets

$$729 \int (1-sin^2 \theta)^{5/2} cos \theta d\theta$$

I'm not sure where to go from here. Do I combine everything algebraically and then do a u sub? Or do I integrate by parts here?

2. Jan 26, 2009

### AEM

Well, you could remember that $$(1 - sin^2 \theta) = cos^2 \theta$$ so that your integrand becomes $$cos^6 \theta$$ and then look that integral up in a table of integrals, or look up a reduction formula for $$cos^6 \theta$$. You'll be able to integrate the expression that results from the reduction formula.

3. Jan 27, 2009

### gabbagabbahey

Careful; $$(\cos^2 \theta)^{5/2}=|\cos^5 \theta|\neq\cos^5 \theta$$ in general.

4. Jan 27, 2009

### AEM

You are, of course, correct. I should have been more careful.