Originally Posted by

**psu1024** Thanks for the help from above posters, I appreciate the explanations. However, the original problem I had trouble with was

$\displaystyle \int x\sqrt{81-9x^2} dx $,

with x times the sqrt, not x divided by the sqrt, which is part of what was tricky for me. (I think it was the problem I had with the Latex syntax when I originally posted that might have caused the mix-up.) So I've drawn my triangle, and I'm pretty sure that $\displaystyle x=3 sin(\theta)$ and $\displaystyle dx=3 cos(\theta) d\theta$. I think that in theory, I should be able to figure out the rest and replace everything back into the equation, but I'm just stuck and unable to work through to get an answer. Is there something I am doing wrong? If not, how should I work this to get an answer?