You have the right idea . . . just carry it through.
And we have: .
. . . . . . . . .
. . Then: .
so, the problem is integral sq rt (1-4x^2)dx
i really didn't know how to start, other than the fact that it looks sort of like the a^2-x^2 form of trig sub (in which i think you're supposed to make x= asintheta)
so, i tried that, but i got lost/confused not long after