- #1
phyzmatix
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Homework Statement
Use trigonometric substitution to evaluate
[tex]\int{\frac{x^2}{\sqrt{9-x^2}}}dx[/tex]
The Attempt at a Solution
Let [tex]x=3\sin\theta[/tex]
then [tex]dx=3\cos\theta d\theta[/tex]
[tex]\int{\frac{x^2}{\sqrt{9-x^2}}}dx[/tex]
[tex]=\int{\frac{9\sin^2\theta}{3\sqrt{1-\sin^2\theta}}}\ 3\cos\theta \ d\theta[/tex]
[tex]=\int{\frac{9\sin^2\theta}{3\sqrt{\cos^2\theta}}}\ 3\cos\theta \ d\theta[/tex]
[tex]=\int{9\sin^2\theta \ d\theta}[/tex]
[tex]=\frac{9}{2} \int{(1-\cos2\theta)}\ d\theta[/tex]
let [tex]w=2\theta[/tex]
then [tex]dw=2\ d\theta[/tex]
[tex]=\frac{9}{4} \int{(1-\cos w)}\ dw[/tex]
[tex]=\frac{9}{4}[w-\sin w] + c \ \mbox{(substituting everything back in)}[/tex]
[tex]=\frac{9}{4}[2\sin^{-1}(\frac{x}{3})-\sin(2\sin^{-1}(\frac{x}{3})]+c[/tex]
Is this correct so far? And if so, now what?
I'm stumped
Thanks!
phyz