Câu hỏi: Nếu $\int\limits_{0}^{3}{f\left( x \right)\text{d}x}=4$ thì $\int\limits_{0}^{3}{3f\left( x \right)\text{d}x}$ bằng A. $36$. B. $12$. C. $3$. D. $4$.
Ta có $\int\limits_{0}^{3}{3f\left( x \right)\text{d}x}=3\int\limits_{0}^{3}{f\left( x \right)\text{d}x}=3.4=12$.