This can be happening for negative values of theta since the range of arcsin is [-pi/2,pi2] and that of arccos is [0,pi] . You can try to first calculate tan(theta) and then calculate arctan. Something like this might help:
a = sin(x)
b = cos(y)
c = tan(y)
d = atand(c) % or use atan2d as d = atan2d(sin(x),cos(x))
