I'm not strong with this, i need some help:
2(1-sin(b)sin(c)) = cos^2(b) + cos^(c) when b+c = 180
sin(angle) gives the vertical co-ordinate in a circle of radius 1, centre (0,0).
Therefore your equation becomes
Using similar logic
allows you to write the entire equation in terms of any one of the four of
sinc, sinb, cosc, cosb