Question

(1-sina)*(1+sina)/cosa

Answer 1

По формуле a²-b²=(a-b)(a+b)
(1-sinα)(1+sinα)/cosα=
=(1-sin²α)/cosα=cos²α/cosα=cosα

Answer 2

$\frac{(1-sina)(1+1sina)}{cosa} = \frac{1-sin^2a}{cosa} = \frac{sin^2a+cos^2a-sin^2a}{cosa} = \frac{cos^2a}{cosa} =cosa$