Đáp án:
$\begin{array}{l}
\int {\frac{2}{{x.\ln x}}dx} \\
= \int {\frac{2}{{\ln x}}.\frac{1}{x}dx} \\
= \int {\frac{2}{{\ln x}}.d\left( {\ln x} \right)} \\
= 2.\int {\frac{1}{{\ln x}}d\left( {\ln x} \right)} \\
= 2.\ln \left( {\ln x} \right) + C
\end{array}$