#1**+2 **

Evaluating each

cos ( x + y) - cos (x - y) = [ cosxcosy - sinxsin y] - [ cosxcosy + sinxsiny ] = -2sinxsiny

Not an identity

sin(x + y) - sin (x - y) = [sinxcosy + sinycosx] - [ sinxcosy - sin ycosx ] = 2cosxsiny

Identity

The third is not an identity [ see the first one ]

sin (x - pi) = sinxcospi - sin pi cosx = -1sin x - 0 = -sin x

Identity

CPhill Apr 25, 2019