Question

(1+sin y)(1+sin (-y))=cos^2 y)

Answer 1

sin (-x) = - sin x
sin^2+cos^2=1

(1+siny)(1-siny)=cos^2y 
1-sin^2=cos^2y
cos^y=cos^2

Answer 2

$(1+siny)(1+sin(-y))=cos^2y\\\\L=(1+siny)(1-siny)=1^2-sin^2y=1-sin^2y=cos^2y\\\\R=cos^2y\\\\L=R$


$sin(-x)=-sinx\\\\sin^2x+cos^2x=1\to1-sin^2x=cos^2x\\\\(a+b)(a-b)=a^2-b^2$