3^a=4^b=6^c=k ==>k=1时,都为0；k≠0时,a=log3(k),b=log4(k),c=log6(k)
1/a=logk(3),1/b=logk(4)=2logk(2),1/c=logk(6)=logk(2)+logk(3)
所以1/c=1/a+1/(2b)a=log3(k)为什么会变成1/a=logk(3),用换底公式：loga(M)/loga(N)=logN(M)