求lim[ x^(n+1)-(n+1)x+n]/(x-1)^2 x-->1

求lim[ x^(n+1)-(n+1)x+n]/(x-1)^2 x-->1

令:x = 1+t(t->0)
lim(x->1)[ x^(n+1)-(n+1)x+n]/(x-1)^2
=lim(t->0)[(1+t)^(n+1)-(n+1)(1+t)+ n]/t^2
=lim(t->0)[ [ 1 +(n+1)t +(n+1)n/2t^2 + o(t^2)] -(n+1)-(n+1)t + n]/t^2
=(n+1)n/2