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**[SOLVED] Integral with trig substitution**

**1. Homework Statement**

Find [tex]\int(x^3)/\sqrt{x^2-9}[/tex]

**2. Homework Equations**

Trig substitution. sin^2 + cos^2 =1, and other things that you can figure out from that.

Half angle formula, cos^2[tex]\theta[/tex]=(1+cos(2[tex]\theta[/tex]) )*.5

**3. The Attempt at a Solution**

Let x=3*sec[tex]\theta[/tex]

so dx=3*sec[tex]\theta[/tex]*tan[tex]\theta[/tex] d[tex]\theta[/tex]

When I substitute that in and simplify it, I got:

27*[tex]\int(sec^4(\theta) d\theta)[/tex]

And I don't know how to integrate that. Half angle formulas aren't seeming to work.

Thanks!