When you want to find sin^-1 (pi/3) what you're asking is to find some x such that sin x = pi/3. Now note that pi/3 = 1.0472. So you want to find x such that sin x = 1.0472. But -1 <= sin x <= 1, this is the range of the sine function. So there is no value of x for which sin x = 1.0472.
