What is the Formula of Cos (A + B + C) ?

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

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