Question

if cosA +cosB=0 AND sinA+sinB=0, then show that cos2A+cos2B+2cos (A+B)=0

Answer 1

Cos^2 A - sin^2 A + cos^2 B - sin^2 B +2 (cosAcosB-sinAsinB)
(CosA+cosB)^2 - (sinA+sinB)^2
0-0
0