# Homework Help: Trig substitution integral

1. Feb 3, 2010

### montana111

1. The problem statement, all variables and given/known data
evaluate the integral using the indicated trig substitution. sketch the corresponding right triangle.

integral of(1/(x^2 sqrt(x^2 - 9))

2. Relevant equations

integral of(1/(x^2 sqrt(x^2 - 9))

3. The attempt at a solution
at first glance this seemed really easy, and i tried doing it without the given trig substitution of 3sec(theta), but i just got confused. i made the triangle with 3 on the hypotenuse and x on the leg opposite theta. then i said x = 3sin(theta) (because i have a value for the hypotenuse and the opposite) and from there i have no idea what to do. Also, im discouraged because the book tells you to use 3sec(theta) which is driving me insane because i have no idea why i would use sec at all. thanks for you help.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 3, 2010

### sara_87

The hint is infact very useful.

you have:

$$\int\frac{1}{x^2\sqrt{x^2-9}}dx$$

using the substitution x = 3sec$$\theta$$
you will get a simple integral.

substitute this into the integral. BUT... you have to first find dx/d$$\theta$$ in order to substitute something for dx (in terms of d$$\theta$$)

so after the substitution, what do you get?
(You might like to remember also the identity (sec$$\theta$$)^2-1=tan($$\theta$$)^2)

3. Feb 3, 2010

### Staff: Mentor

Since the radical contains x2 - 9, you want x on the hypotenuse and 3 on one leg, and sqrt(x2 - 9) on the other leg.