Cho 2 ∫ 1 [ 4 f ( x ) − 2 x ] d x = 1 . Khi đó 2 ∫ 1 f ( x ) d x bằng:

Đáp án đúng là: A

\[\begin{array}{l}\int\limits_1^2 {\left[ {4f\left( x \right) - 2x} \right]dx} = 1 \Leftrightarrow 4\int\limits_1^2 {f\left( x \right)dx - 2\int\limits_1^2 {xdx} } = 1 \Leftrightarrow 4\int\limits_1^2 {f\left( x \right)dx - 2.} \left. {\frac{{{x^2}}}{2}} \right|_1^2 = 1\\ \Leftrightarrow 4\int\limits_1^2 {f\left( x \right)dx = 4 \Leftrightarrow } \int\limits_1^2 {f\left( x \right)dx = 1} .\end{array}\]