1、-3/2,5/4,-7/8,9/16,.,第n个数是 (-1)^n * (2n+1) / (2n)
2、-x,+x,-x,+x,.,第n个数是 (-1)^n * x
3、+x,-x,+x,-x,.,第n个数是 (-1)^(n+1) * x