cosx-cosy=-2sin[(x+y)/2]sin[(x-y)/2] [cos(x+dx)-cosx]/dx = -2sin(x+dx/2)*sin(dx/2)/dx= -sin(x+dx/2) (sin(dx/2)/(dx/2))dx->0sin(dx/2)/(dx/2) ->1sin(x+dx/2) -> sinx
