条件:lg2=a,lg3=b;
以2为底12的对数=lg12/lg2=(lg3+2lg2)/lg2=(2a+b)/a=2+b/a.
以3为底4的对数=lg4/lg3=2lg2/lg3=2a/b.
a=lg2 b=lg3
log2 12=log2 4*3=log2 4 +log2 3
=log2 2^2 +log2 3
=2+ lg3/lg2
=2+b/a
log3 4 =log3 2^2 =2log3 2 =2lg2/lg3=2a/b