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Câu hỏi: Cho $\int\limits_{0}^{\dfrac{\pi }{2}}{f\left( x \right)\text{d}x}=4$. Khi đó $I=\int\limits_{0}^{\dfrac{\pi }{2}}{\left[ 2f\left( x \right)-\cos x \right]\text{d}x}$ bằng
A. $9$.
B. $7$.
C. $6$.
D. $1$.
A. $9$.
B. $7$.
C. $6$.
D. $1$.
Ta có $I=\int\limits_{0}^{\dfrac{\pi }{2}}{\left[ 2f\left( x \right)-\cos x \right]\text{d}x}=\int\limits_{0}^{\dfrac{\pi }{2}}{2f\left( x \right)\text{d}x}-\int\limits_{0}^{\dfrac{\pi }{2}}{\cos x\text{d}x}=2.4-\left. \sin x \right|_{0}^{\dfrac{\pi }{2}}=7$.
Đáp án B.