F.2 MAths

5 [{cos(90 - θ)} {cos(90 - θ)}] -2 ≡ 3 - 5 [{cosθ} {cosθ}]
LHS = [{cos(90 - θ)} {cos(90 - θ)}] -2
= 5 sinθ sinθ -2
= 5sin2θ – 2
= 5(1 – cos2θ) – 2
= 5 – 5cos2θ – 2
= 3 – 5cos2θ
= RHS
LHS = RHS ∴QED