sin(α-β)sinβ-coscos=4/5
-cos[(α-β)+β]=4/5
cosα=-4/5 α在第二象限
sinα=3/5
tanα=-3/4
tan(π/4+α)=(tanα+tanπ/4)/[1-tanαtanπ/4]
=(1-3/4)/(1+3/4)
=1/7sinα的值是怎么求出来的啊cosα=-4/5α在第二象限sinα>0sin²α+cos²α=1