应用公式：
①log3(M/N)=log3(M)-log3(N)
②log3(Mⁿ)=nlog3(M)
③M^logM(N)=N
2log3(2)-log3(2/9)+log3(8)-5^log5(3)
=2log3(2)-[log3(2)-log3(9)]+3log3(2)-3
=2log3(2)-log3(2)+2log3(3)+3log3(2)-3
=4log3(2)+2-3
=4log3(2)-1