Question

Prove that:
Cos (A+B)=cos A cos B -sin A sin B

Answer 1

cos(A+B)=OS/OR
cos(A+B)=OQ-SQ/OR (OS=OQ-SQ) cos(A+B)=OQ-TP/OR. (TP=SQ)
cos(A+B)=OQ/OP.OP/OR-TP/RP.RP/OR
cos(A+B)=cosAcosB-sinAsinB
because, [cosA=OQ/OP, cosB=OP/OR, sinA=TP/RP/, sinB=RP/OR].
so, cos(A+B)=cosAcosB-sinAsinB