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Câu hỏi: Nếu $\int\limits_{1}^{2}{f\left( x \right)}\text{d}x=3$ và $\int\limits_{2}^{1}{g\left( x \right)}\text{d}x=1$ thì $\int\limits_{1}^{2}{\left[ f\left( x \right)+2g\left( x \right) \right]}\text{d}x$ bằng
A. $5$.
B. $1$.
C. $-1$.
D. $0$.
A. $5$.
B. $1$.
C. $-1$.
D. $0$.
$\int\limits_{1}^{2}{\left[ f\left( x \right)+2g\left( x \right) \right]}\text{d}x=\int\limits_{1}^{2}{f\left( x \right)}\text{d}x+2\int\limits_{1}^{2}{g\left( x \right)}\text{d}x=\int\limits_{1}^{2}{f\left( x \right)}\text{d}x-2\int\limits_{2}^{1}{g\left( x \right)}\text{d}x=3-2=1$.
Đáp án B.