# Integrating Using Trigonometric Substitution

1. Aug 6, 2011

### prosteve037

1. The problem statement, all variables and given/known data

Use the method of trigonometric substitution to evaluate the following:

$\int\frac{x^{2}}{\sqrt{4-9x^{2}}}$

2. Relevant equations

The only relevant equation that I could think of for this one was the trig identity:

$sin^{2}\vartheta + cos^{2}\vartheta = 1$

3. The attempt at a solution

This is what I got from trying this problem:

$\int\frac{x^{2}}{\sqrt{4-9x^{2}}}$

$\sqrt{4-9x^{2}}$ is a difference between two squares, so it's a leg of a right triangle. So I let $4sin\vartheta = \sqrt{4-9x^{2}}$ to satisfy the definition of sine (Opposite Side over Hypotenuse).

Then, I let $x^{2} = \frac{4}{9}cos\vartheta$ to satisfy the definition of cosine.

Differentiating this same function with respect to $x$, I got:

$\frac{\frac{-2}{9}sin\vartheta}{\sqrt{\frac{4}{9}cos\vartheta}}$

Substituting these into the original function:

$\int{\frac{\frac{4}{9}cos\vartheta}{4sin\vartheta} \times \frac{\frac{-2}{9}sin\vartheta}{\sqrt{\frac{4}{9}cos\vartheta}}d\vartheta}$

$\int{\frac{\frac{4}{9}cos\vartheta}{4sin\vartheta} \times \frac{\frac{-2}{9}sin\vartheta}{\frac{2}{3}\sqrt{cos\vartheta}}d\vartheta}$

$\int{\frac{1}{9} \times \frac{cos\vartheta}{sin\vartheta} \times \frac{-1}{3} \times \frac{sin\vartheta}{\sqrt{cos\vartheta}}d\vartheta}$

$\frac{-1}{27}\int{\frac{cos\vartheta}{\sqrt{cos\vartheta}}d\vartheta}$

At this point, I got stuck and didn't know what to do. I tried using a modified version of the identity above and just went on to see if I could get anywhere but failed.

Thanks

2. Aug 6, 2011

### GreenPrint

they hypotenuse is actually 2, remember c^2=a^2 + b^2, the other leg is actually 3x

Last edited: Aug 6, 2011
3. Aug 6, 2011

### Bohrok

I can't really see how you got these substitutions... A standard substitution for this problem would be $x = \frac{2}{3}\cos\vartheta$

4. Aug 6, 2011

### SammyS

Staff Emeritus
It may help you to indicate that x is the variable of integration, by including dx in your integral: $\displaystyle \int\frac{x^{2}}{\sqrt{4-9x^{2}}}\,dx\,.$

To elaborate on GreenPrint's post:

If $\sqrt{4-9x^{2}}$ is a leg of a right triangle, then the hypotenuse would likely be 2 and the other leg 3x.

As you say, sine is opposite over hypotenuse, so either $\displaystyle \sin(\theta)=\frac{3x}{2}$ or $\displaystyle \sin(\theta)=\frac{\sqrt{4-9x^{2}}}{2}\,.$

cos(θ) is the other.

5. Aug 7, 2011

### Harrisonized

Just making info easier to understand.

[PLAIN]http://img856.imageshack.us/img856/4011/derpto.png [Broken]

Make sure you find dx to get dθ.

Last edited by a moderator: May 5, 2017
6. Aug 7, 2011

### HallsofIvy

Staff Emeritus
No, you must have [itex]x= (2/9)cos(\vartheta)[/tex].

7. Aug 7, 2011

### prosteve037

Oh wow. Okay. Yeah I know what I did wrong now. Thanks everyone!

I'm still a little confused about choosing which leg to be the radical though. Does it matter? I was taught to just put the radical term on the opposite side of the reference angle. But are there instances in which this won't work?

8. Aug 7, 2011