2.按 -N 定义证明:-|||-(1) lim _(narrow infty )dfrac (n)(n+1)=1 ;-|||-(2) lim _(narrow infty )dfrac (3{n)^2+n}(2{n)^2-1}=dfrac (3)(2) ;-|||-(3) lim _(narrow infty )(n)^n=0 ;-|||-(4) lim _(narrow infty )sin dfrac (pi )(n)=0 ;-|||-(5) lim _(narrow infty )dfrac (n)({a)^n}=0(agt 1).