c=log42(56)=lg56/lg42=(3lg2+lg7)/(lg2+lg3+lg7)
ab=lg7/lg2,lg7=ablg2.
a=lg3/lg2,lg3=alg2.
代入：c=(3lg2+ablg2)/(lg2+alg2+ablg2)
=(3+ab)/(1+a+ab).