We know:
cos(a - b) = cos(a)cos(b) + sin(a)sin(b)
cos(a + b) = cos(a)cos(b) - sin(a)sin(b)
cos(a - b) - cos(a + b) = 2sin(a)sin(b)
L = cos(a)cos(b) + sin(a)sin(b) - (cos(a)cos(b) - sin(a)sin(b))
= cos(a)cos(b) + sin(a)sin(b) - cos(a)cos(b) + sin(a)sin(b)
= 2sin(a)sin(b) = R
