Aidez moi sil vous plait

Ex 3 :
(1-cos(x))(1+cos(x))=1²-cos²(x)=sin²(x)
donc (1-cos(x))/sin(x)=sin(x)/(1+cos(x))

(1-sin(x))(1+sin(x))=1²-sin²(x)=cos²(x)
donc cos(x)/(1-sin(x))=(1+sin(x))/cos(x)

(cos(x))^4-(sin(x))^4=(cos²(x)-sin²(x))(cos²(x)+sin²(x))
                              =cos²(x)-sin²(x)
                              =cos²(x)-(1-sin²(x))
                              =2cos²(x)-1