f[ln(3x)] = ln(3x) ^3分布求导，u = ln(3x), u^3导数=3u^2令v =3x，u = ln v， u导数=1/vv' = 3然后全部乘起来得 3u^2 * 1/v * 3u和v代入，答案C
=3(ln(3x)x)^2 * 3/(3x)=3(ln(3x)x)^2 / x
