Cho (4 f(x) -2x ) dx = 1 khi đó f (x) dx bằng

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