Ответ:

Объяснение:4)=cos(2·3π/8)=cos3π/4=cos(π-π/4)=-cosπ/4= -√2 /2.

3)=(cosα-2sinα)/(cosα+sinα) - (2-cos²α)/(cos²α-sin²α)=

((cosα-2sinα)(cosα--3sinαsinα) -(2-cos²α)) /(cos²α-sin²α) =

(cos²α-cosα·sinα-2sinα·cosα+2sin²α -2+cos²α) /cos2α=

( - 3sinα·cosα)/cos2α=(-3/2·2sinα·cosα)/cos2α=(-3/2sin2α)/cos2α=-1,5tg2α.