2 sin ( 315derajat + x ). sin ( 45derajat + x ) = - cos 2x

2.sin(315° + x).sin(45° + x) = -cos 2x
2.sin(360°-(45°-x)).sin(90°-(45°-x)) = -cos 2x
2.(-sin(45°-x)).cos(45°-x) = -cos 2x
-(2.sin(45°-x). cos (45°-x)) = -cos 2x
-sin(90-2x) = -cos 2x
cos 2x = cos 2x
*terbukti*