任意给定正数a,取正整数N=max{4,24/a}
当n>=N时,1/n<2/n<1/2
|n^2/2^n|
=n^2/2^n
=6/[(1-1/n)(1-2/n)]*1/n
<6/(1/2*1/2)*1/n
=24/n
所以limn^2/2^n=0