an=(-1)^nan-1=(-1)^(n-1)an/an-1=-1an是以-1为首,-1为公比的等比数列.Sn=-1*(1-(-1)^n)/(1-(-1))=[(-1)^n-1]/2S(2n+1)=-1(1-(-1)^(2n+1))/(1-(-1))=-1(1+(-1)^(2n))/(2)=-(1+1)/2=-1
