Cho \(\int\limits_0^1 {\left( {1 + 3x} \right)f’\left( x \right){\rm{d}}x} = 2019; 4f\left( 1 \right) – f\left( 0 \right) = 2020\). Tính \(\int\limits_0^{\frac{1}{3}} {f\left( {3x} \right){\rm{d}}x} \)
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Lời giải:
Báo saiĐặt \(\left\{ \begin{array}{l}u = 1 + 3x\\{\rm{d}}v = f’\left( x \right){\rm{d}}x\end{array} \right. \Rightarrow \left\{ \begin{array}{l}{\rm{d}}u = 3{\rm{d}}x\\v = f\left( x \right)\end{array} \right.\)
\(\begin{array}{l}\int\limits_0^1 {\left( {1 + 3x} \right)f’\left( x \right){\rm{d}}x} = 2019\\ \Leftrightarrow \left( {1 + 3x} \right).\left. {f\left( x \right)} \right|_0^1 – \int\limits_0^1 {3.f\left( x \right){\rm{d}}x} = 2019\\ \Leftrightarrow 4f\left( 1 \right) – f\left( 0 \right) – 3\int\limits_0^1 {f\left( x \right){\rm{d}}x} = 2019\\ \Leftrightarrow \int\limits_0^1 {f\left( x \right){\rm{d}}x} = \frac{1}{3}\end{array}\)
Ta có: \(\int\limits_0^{\frac{1}{3}} {f\left( {3x} \right){\rm{d}}x} = \frac{1}{3}\int\limits_0^1 {f\left( t \right){\rm{dt}}} = \frac{1}{3}.\frac{1}{3} = \frac{1}{9}\).