Question

(cos215° - sin215°)·(cos215° + sin215°)

Answer 1

(-сos35+sin35)(-cos35-sin35)=-(sin35-cos35)(sin35+cos35)=-(sin²35-cos²35)=
=-(-cos70)=cos70

Answer 2

$(cos215-sin215)*(cos215+sin215)=cos ^{2} 215-sin ^{2} 215= \\ =cos ^{2} (180+35)-sin ^{2}(180+35)=cos ^{2} 35-sin ^{2} 35= \\ =cos70$