log2(3)=a
则有：log3(2)=1/a
由于：3^b=7
则：log3(7)=b
则：log12(56)
利用换底公式：
log12(56)
=log3(56)/log3(12)
=[log3(7)+log3(8)]/[log3(3)+log3(4)]
=[b+log3(2^3)]/[1+log3(2^2)]
=[b+3log3(2)]/[1+2log3(2)]
=[b+3/a]/[1+2/a]
=[ab+3]/[a+2]