答：
1）
lg2=a,lg3=b
log12(5)
=lg5 / lg12
=lg(10/2) / lg(2*2*3)
=(lg10-lg2) / (lg2+lg2+lg3)
=(1-a) /(2a+b)
2)
log2(3)=a,log3(7)=b
因为：log3(7)=log2(7)/log2(3)
所以：b=log2(7)/a
所以：ab=log2(7)
log14(56)
=log14(14*4)
=log14(14)+log14(4)
=1+log2(4)/log2(14)
=1+2/log2(2*7)
=1+2/ [log2(2)+log2(7)]
=1+2/(1+ab)