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Câu hỏi: Cho hàm số $f\left( x \right)=\cos \dfrac{x}{2}$. Khẳng định nào dưới đây đúng?
A. $\int{f\left( x \right)\text{d}x}=2\sin \dfrac{x}{2}+C$.
B. $\int{f\left( x \right)\text{d}x}=-2\sin \dfrac{x}{2}+C$.
C. $\int{f\left( x \right)\text{d}x}=\sin \dfrac{x}{2}+C$.
D. $\int{f\left( x \right)\text{d}x}=-\sin \dfrac{x}{2}+C$.
A. $\int{f\left( x \right)\text{d}x}=2\sin \dfrac{x}{2}+C$.
B. $\int{f\left( x \right)\text{d}x}=-2\sin \dfrac{x}{2}+C$.
C. $\int{f\left( x \right)\text{d}x}=\sin \dfrac{x}{2}+C$.
D. $\int{f\left( x \right)\text{d}x}=-\sin \dfrac{x}{2}+C$.
Ta có ${{\left( 2\sin \dfrac{x}{2} \right)}^{\prime }}=\cos \dfrac{x}{2}$ nên $\int{f\left( x \right)\text{d}x}=2\sin \dfrac{x}{2}+C$.
Đáp án A.