sina=4/5,π/2＜a＜π,所以cosa=-3/5;
cos(a-β）=5/13 ,2π＜a-β＜3π所以sin(a-β）=12/13
cosβ=cos[a-(a-β）]=(5/13)×(-3/5)+(4/5)(12/13)=33/65