[2/5-(-3/5)]/4=1/4
-3/5+1/4=-7/20
-7/20+1/4=-2/20=-1/10
-1/10+1/4=3/20
3/20+1/4=8/20=2/5
每相邻两个数差相等
那三个数是:-7/20,-1/10,3/20