s in quadrant II (第二象限) and t in quadrant I (第一象限)sins=√[1-(coss)^2]=√[1-(15/17)^2=8/17cost=√[1-(sint)^2]=√[1-(4/5)^2]=3/5 (注:这里有问题,第一象限sint怎么为负值?)我这里改为正值sin(s-t)=sinscost-sintcoss=(8/17)*(3/5)-(4/5)*(-15/17)=84/85