Câu hỏi: Nếu $\int\limits_{1}^{9}{f\left( x \right)\text{d}x}=3$ thì $\int\limits_{1}^{9}{3f\left( x \right)\text{d}x}$ bằng A. $1$. B. $9$. C. $2$. D. $6$.
Ta có $\int\limits_{1}^{9}{3f\left( x \right)\text{d}x}=3\int\limits_{1}^{9}{f\left( x \right)\text{d}x}=9$.
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