how to prove cos(pi/4 – theta)cos(pi/4- fi) -sin(pi/4 – theta)sin(pi/4 +fi) = sin(theta + fi)

Firstly multiply and divide by 2 in LHs.1/2[{2cos(pi/4-theta)cos{(pi/4-fi)}-{2sin(pi/4-theta)sin(pi/4-fi}]Then apply identity of 2casAcosB and 2 sin AsinB in the equationFinally u will get 1/2{2[cos(pi/2-theta-fi)]}Which gives u sin{theta +fi}