log以3为底6的对数
=lg6/lg3
=(lg2+lg3)/lg3
=(a+b)/b
log以12为底5的对数
=lg5/lg12
=(1-lg2)/lg12
=(1-lg2)/(lg3+lg4)
=(1-lg2)/(lg3+2lg2)
=(1-a)/(b+2a)