log(a^m,N^n) = lg(N^n)/lg(a^m) = nlg(N)/mlg(a) =n/m lg(N)/lg(a) = n/m log(a,N)
log(a,b) = lg(b)/lg(a) = 1/(lg(a)/lg(b) = 1/log(a,b)
不是都是很简单的变换么?