Prove (sin x - sin y)/(cos x - cos y) = - cot ((x+y)/2)
(sin x - sin y) / (cos x - cos y) = - cot ((x+y)/2)
( 2cos[(x + y)/2]*sin [(x - y)/2] ) / ( -2sin [(x + y)/2 ]* sin [(x - y)/2] ) =
2cos[(x + y)/2] / -2sin [(x + y)/2] =
- (cos[(x + y)/2] / sin [(x + y)/2] )=
- cot [ (x + y)/2 ]
