死办法是这样的
lim(x->∞) [2^(n+1)+3^(n+1)]/(2^n+3^n)=lim(x->∞)(2*2^n+2*3^n+3^n)/(2^n+3^n)=2+lim(x->∞) 3^n/(2^n+3^n)=2+lim(x->∞) 1/[1+(2/3)^n]=2+1=3
