Đáp án:
$\begin{array}{l}
I = \int\limits_2^4 {f\left( {\frac{x}{2}} \right)dx} \\
= \int\limits_2^4 {f\left( {\frac{x}{2}} \right).2.\frac{1}{2}dx} \\
= 2.\int\limits_2^4 {f\left( {\frac{x}{2}} \right)d\frac{x}{2}} \\
= 2.\int\limits_1^2 {f\left( u \right)du} \\
= 2.3\\
= 6
\end{array}$