Solution :
The formula of cos(A + B + C) is cos A cos B cos C – sin A sin B cos C – sin A cos B sin C – cos A sin B sin C.
Proof :
We have,
cos (A + B + C) = cos ((A + B) + C) = cos (A + B) cos C – sin (A + B) sin C
cos (A + B + C) = (cos A cos B – sin A sin B) cos C – (sin A cos B + cos A sin B) sin C
cos (A + B + C) = cos A cos B cos C – sin A sin B cos C – sin A cos B sin C – cos A sin B sin C
