A’= A(cosx)^2 + 2Bcosxsinx + C(sinx)^2

C’= A(sinx)^2 – 2Bcosxsinx + C(cosx)^2

I need to prove that (A’)(C’) is equal to AC – B^2

If tried multiplying both together but I cant seem to find the right trig identities or how to simplify it.

any steps or hints would be Great