观察下列各数：-(1/2),2/3,-(3/4),4/5,-(5/6)…

设第n项为an.a1=-1/2=(-1)^1[1/(1+1)]a2=2/3=(-1)^2[2/(2+1)]a3=-3/4=(-1)^3[3/(3+1)]a4=4/5=(-1)^4[4/(4+1)]a5=(-1)^5[5/(5+1)]…………an=(-1)^n[n/(n+1)] 规律：第n项分子为n,分母为n+1,奇数项为负,偶数项为正....