Câu hỏi: Nếu $\int\limits_{2}^{5}{f\left( x \right)dx=4}$ thì $\int\limits_{2}^{5}{5f\left( x \right)\text{d}x}$ bằng A. $20$. B. $15$. C. $9$. D. $1$.
Ta có: $\int\limits_{2}^{5}{5f\left( x \right)\text{d}x}=5\int\limits_{2}^{5}{f\left( x \right)\text{d}x}=5.4=20$.
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