log2 3=a ----->log3 2=1/a
log3 7=b
log3 (56)=log3(7*2^3)=log3(7)+3log3(2)=b+3/a=(ab+3)/a
log3(14)=log3(7*2)=log3(7)+log3(2)=b+1/a=(ab+1)/a
所以,由换底公式,log14 56=log3 (56)/log3(14)=(ab+3)/(ab+1)