a=log2(3)=lg3/lg2b=log3(7)=lg7/lg3可得：ab=lg7/lg2log42(56)=lg56/lg42=lg(7x8)/lg(7x2x3)=(lg7+3lg2)/(lg7+lg2+lg3) 分子分母同时除以lg2得：=(lg7/lg2+3)/(lg7/lga+1+lg3/lg2)=(ab+3)/(ab+1+a)