设段数是偶数6 x1,x2,x3,x4,x5,x6
a=[(x4+x5+x6 )- (x1+x2+x3)]/9T^2
设段数是奇数5 x1,x2,x3,x4,x5
a=[(x4+x5 )- (x1+x2)]/6T^2