[2^(N+1)+3^(N+1)] / (2^N+3^N)
=(2*2^N+3*3^N) / (2^N+3^N)
分子分母同除以 3^N：
=[2*(2/3)^N+3] / (2/3)^N+1]
当N趋近+∞时：
=（2*0+3）/（0+1）=3