數(shù)列排列規(guī)律：分子為項數(shù),分母為項數(shù)+1,奇數(shù)項為負(fù),偶數(shù)項為正.
n為奇數(shù),第n個數(shù)與第n+1個數(shù)之和=-n/(n+1) +(n+1)/(n+2)
=[-n(n+2)+(n+1)²]/[(n+1)(n+2)]
=(-n²-2n+n²+2n+1)/[(n+1)(n+2)]
=1/[(n+1)(n+2)]
=1/(n+1) - 1/(n+2)