第n个数与第n+1个数的和（n为奇数）
=-n/(n+1)+(n+1)/(n+2)
=(n+1)/(n+2)-n/(n+1)
=[(n+1)²-n(n+2)] / [(n+1)(n+2)]
=(n²+2n+1-n²-2n)/(n²+3n+2)
=1/(n²+3n+2)