cosα等于-4/5,α是第三项的角求sin（3π/4-α）,cos（π/4+2α）的值

cosα=-4/5,α是第三象限的角sinα=-3/5cos2α=2cosα*cosα-1=7/25sin2α=2sinαcosα=24/25sin(3π/4-α)=sin3π/4*cosα-cos3π/4*sinα=-7√2/10cos(π/4+2α)=cosπ/4*cos2α-sinπ/4*sin2α=-17√2/50