That is a very good question
Look Wade...
If you faced it in the form (x^2+a)^0.5 , you would use the substitution of x=a*tan(y)
If you faced it in the form (x^2-a)^0.5 , you would use the substitution of x=a*sec(y)
If you faced it in the form (a-x^2)^0.5 , you would use the substitution of x=a*sin(y) or even x=a*cos(y)
It would be better if you provided an example ...
I am with you if you faced any difficulties!
:)