(2^n-3^n)/[2^(n+1)+3^(n+1)]
=(2^n-3^n)/(2*2^n+3*3^n)
上下除3^n
=[(2/3)^n-1]/[2*(2/3)^n+3]
n→∞
(2/3)^n→0
所以极限=(0-1)/(2*0+3)=-1/3