# Why cant i use substitution for this?

1. Dec 6, 2011

### schapman22

Why doesn't substitution work to find 0∫2 sqrt(4-x^2). I kn ow you can find the integral in other ways. I am just curious why regular substitution won't work. Thank you in advance.

2. Dec 6, 2011

### spamiam

If you make the substitution $u=x^2$ then du = 2x dx. But you don't have a factor of 2x in the integrand. Or were you thinking of a different substitution? In any case, a trigonometric substitution would work better.

3. Dec 6, 2011

### LCKurtz

But substitution does work. Make the substitution x = 2cos(θ). Isn't that a "substitution"?