# Cal 2 integral / trig substitution

1. Sep 14, 2011

### ryantruran2

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

I am asked to prove the following statement is correct

integral (sqrt(a^2+x^2))/x dx = sqrt(a^2+x^2)-a log(a (sqrt(a^2+x^2)+a))+ C

2. Relevant equations

x = atanθ
dx = (asecθ)^2

tan^2+1 = sec^2

3. The attempt at a solution

got down to a (sec^2 θ a(√sec^2)dθ)/atanθ

I plugged into wolfram and immediately got something involving csc in the steps and im not sure where it came from. Just beginning these trig substitutions in class.

Last edited: Sep 14, 2011
2. Sep 14, 2011

### lanedance

$$x = atan \theta$$
$$dx = a sec^2 \theta d\theta$$

so subbing into the integral you get (might want to check the steps)
$$\int \frac{sqrt{a^2+x^2}}{x}dx = \int \frac{\sqrt{a^2+a^2tan^2 \theta}}{atan\theta} a sec^2 \theta d\theta = \int \frac{\sqrt{a^2sec^2\theta}}{tan\theta} sec^2 \theta d\theta = \int \frac{a}{cos\theta}\frac{cos\theta}{sin\theta} \frac{1}{cos^2 \theta} d\theta = \int a\frac{1}{sin\theta} \frac{1}{cos^2 \theta} d\theta$$

now can you make another substitution?

3. Sep 15, 2011

### ryantruran2

would you use

U= secθ
dU =sec(θ)tan(θ)