1^ste manier :
cos 285°.cos 195° = cos (285°−180°).cos (195°−180°) = cos 105°.cos 15° = − cos 75°.cos15° = − sin 15°.cos 15° = − .2sin15°.cos15 = − sin30°= −   = ...
2^de manier :
cos 285°.cos 195° = sin(90°−285°).cos195° = − sin195°.cos195°
= −   2 sin195°.cos195° = −  sin 390° = −  sin 30° = −   = ...