# Trigonometric Substitution

1. Feb 8, 2012

### basenne

I was messing around proving the area of a circle using trigonometric substitution. However, I ended up with area = -πr^2.

In my integral I ended up using trigonometric substitution and setting x = r*cos(theta)

However, this yields x = -r*sin(theta)*d(theta).

When that's substituted back into my integral, I ended up with the negative value for area. Why is it that if I were to use a different angle theta and express x = r*sin(theta) that I end up with the correct area?

Thanks a lot, as always.

2. Feb 9, 2012

### tiny-tim

hi basenne!

it's probably your limits of integration that were wrong …

what exactly was your integral?​