sin (A + B) = sin A cos B + cos A Sin B
Cos (A - B) = cos A cos B + sin A sin B
=> (SinACosB+ CosASinB) (CosACosB +SinASinB)
=>SinACosACos^2B+Sin^2ACosBSinB+Cos^2A...
=>SinACosA(Cos^2B+Sin^2B) +SinBCosB(Sin^2A+Cos^2A)
we know that Sin^2+Cos^2=1
=>SinACosA(1)+SinBCosB(1)
=SinACosA+SinBCosB
Proved
