lim(3^n+2^2n）/（3^(n+1)+2^(2n+1)）
= lim(n-->∞)[(3^n+4^n)/(3*3^n+2*4^n)]
= lim(n-->∞)[(3/4)^n+1]/[3*(3/4)^n+2*] (分子分母同时除以4^n)
=(0+1)/(0+2)
=1/2