(1+sin y)(1+sin (-y))= cos^2y
Sin (-y)=-sin ysin²y+cos²y=1 ⇒ cos²y=1-sin²yTherefore:(1+sin y)(1+sin(-y))=cos²y(1+sin y)(1-sin y)=cos²y1-sin²y=cos ²ycos²y=cos²yAnswer: it is true this identity .
