find the value for arctan2(sqrt(a),sqart(1-a))
bala08ap still needs to confirm this but I believe that "arctan2(x, y)" is commonly used, in the computer languages C++ and Java, to mean "arctangent of y/x". That is, "y" is the "opposite side" and "x" is the "near side" of a right triangle having angle "arctan2(x, y)".