a,b属于（0.π/2)
sina=4/5
cosa=√(1-sin^2a)=3/5
cos(a+b)
=cosacosb-sinasinb
=3/5cosb-4/5sinb=-3/5
3cosb-4sinb=-3
3cosb=4sinb-3 平方一下
9cos^2b=9(1-sin^2b)=16sin^2b-24sinb+9
25sin^2b-24sinb=0
sinb(25sinb-24)=0
sinb=0(舍去)sinb=24/25