bn-bn-1=2n-6
b2-b1=-4
b3-b2=-2
b4-b3=0
……
bn-bn-1=2n-6 叠加
bn-b1=-4-2+0+2+……+(2n-6)=(2n-10)*(n-1)/2
bn-b1=n^2-6n+5
bn=n^2-6n+5+b1 缺b1的值