设α=arcsin(-4/5)β=arc cos(-3/5)
sinα=-4/5 cosβ=-3/5
-π/2<α<0 所以cosα>0 cosα=3/5
π/2<β<π 所以sinβ>0 sinβ=4/5
cos(α-β)
=cosαcosβ+sinαsinβ
=3/5*(-3/5)+(-4/5)*4/5
=-1