Hi, my teacher did the following step, and I cant understand it really:

We have A = b $(With limits from 0 to a) Sqroot [1-x^2 / a^2 ] dx

he got the common denominator as following: a^2 -x^2 / a^2

Now he suddenly wrote it like this ....

b/a $ Sqroot [a^2 - x^2] dx My queston is how did he do this step??