How to Rotate a Surface Area about the Y-Axis?

[Tarpie]

Greetings,

y=x²/4 - ln(x)/2 from 1=<x<=2

rotated about the y-axis.

I did the equation rotating about the x-axis via 2pi* integral (f(x)*sqrt(1+f'(x)^2)) dx
with dy/dx = x/2 - 1/2x

but the question calls for rotation about y and i can't seem to rearrange the equation to isolate for x in terms of y.

Any ideas?

Thanks

 

[Mark44]

Closed. Homework-type questions should be posted in the appropriate Homework sections.