Answer and explanation
(1+sinθ)(1-sinθ)=cos²θ
We are to prove that the left hand side is equal to the right hand side.
(1+sinθ)(1-sinθ) = 1(1-sinθ) + sinθ(1-sinθ)
= 1 - sinθ + sinθ - sin²θ
= 1 - sin²θ
From the trigonometric identity sin²θ + cos²θ = 1,
1 - sin²θ = cos²θ