sin(2u) = 2sin(u)cos(u)
sin(2sin^-1(x)) = 2sin(sin^-1(x))cos(sin^-1(x)) = 2xcos(sin^-1(x))
let y = sin^-1 x
x =siny
sin^2(y) = 1- cos^2(y)
cos^2(y) = 1 - sin^2(y)
cos^2(y) = 1 -x^2 >= 0 ( since anything^2 is always positive)
+/-cos(y) = sqrt(1-x^2)
sin(2sin^-1(x)) = 2sin(sin^-1(x))cos(sin^-1(x)) = 2xcos(sin^-1(x)) =+/- 2xsqrt(1-x^2)
Hopefully I didn't mess up somewhere.