log9(x+5）
=lg(x+5)/lg9
=lg(x+5)/(2lg3)
=1/2*lg(x+5)/lg3
=½log3 (x+5)log9(x+5）=lg(x+5)/lg9这一步什么意思？我是准高一log9(x+5）=lg(x+5)/lg9这个是换底公式log a (b)=lgb/lga