1.由cosα=-12/13,α∈(π/2,π)得 sinα=5/13,tanα=-5/12tan(α-π/4)=(tanα-tanπ/4)/(1+tanαtanπ/4)=(-5/12-1)/(1+(-5/12)) =-17/72..根据sin（45°-α）=-2/3 求出cos（45°-α）= √5 /3cos（2α）=sin（90...