sina=-3/5,a∈(pai,3pai/2)
cosa小于0
(sina)^2+(cosa)^2=1
cosa=-4/5
cos(a-pai/4)=cosacos(pai/4)+sinasin(pai/4)
=(-4/5)((根2)/2)+(-3/5)((根2)/2)
=-7(根2)/10