log(a)b.log(b)c.log(c)a=1
证明：
∵log(a)b.log(b)c.log(c)a=log(a)b.(log(a)c/log(a)b).(1/log(a)c)=1
∴log(a)b.log(b)c.log(c)a=1
注：上等式换底后约去分式中 log(a)b 和 log(a)c