Respuesta :

Sin (-y)=-sin y
sin²y+cos²y=1    ⇒ cos²y=1-sin²y
Therefore:
(1+sin y)(1+sin(-y))=cos²y
(1+sin y)(1-sin y)=cos²y
1-sin²y=cos ²y
cos²y=cos²y

Answer: it is true this identity .