Originally Posted by

**infraRed** Hi. I'm somewhat frustrated because of a homework question. I don't know the mathhelpforum graphical formatting code, so I'm using '[' to represent that integral S-like symbol:

[ (dx / sqrt(7 - x^2)

I guess that can be translated into...

[ (7 - x^2)^(-1/2) dx

So, my natural inclination is to let u = 7 - x^2. But I do not see 2x elsewhere in the integral. So do I insert it? This doesn't look right:

[ (7 - x^2)^(-1/2) dx = [ u^(-1/2)2xdu = 2 [ u^(-1/2)xdu

Or is it...??? What am I supposed to do with that extra x?