# Trouble grasping surface area formula variations

In the surface area formulas above, I understand that the f(x) will just be what f(x) (aka y) equal (e.g. for y=x+4, f(x) in that formula will be x+4) but I do not understand what h(y) will be. Will it just be f(x) in terms of y? (e.g. for y = x+4, h(y) = y-4)

-
Yes, you are correct. – Matt Jul 7 '11 at 16:56
Rotate your coordinate system by 90 degrees, then your height becomes the distance from the function to the y-axis. – M.B. Jul 7 '11 at 17:13
@Matt: You are right. I think of it as reflection in the line $y=x$, thus interchanging the roles of $x$ and $y$. So just express $x$ in terms of $y$, like you did. – André Nicolas Jul 7 '11 at 17:46