Cho $F\left( x \right)=\int{\cos 2x\text{d}x}$,biết rằng $F\left(...

\)">F\left( x \right)=\int{\cos 2x\text{d}x}=\dfrac{1}{2}\sin 2x+CF\left( \dfrac{\pi }{4} \right)=3\Leftrightarrow \dfrac{1}{2}\sin \dfrac{\pi }{2}+C=3\Leftrightarrow C=\dfrac{5}{2}F\left( \dfrac{\pi }{12} \right)=\dfrac{1}{2}\sin \dfrac{\pi }{6}+\dfrac{5}{2}=\dfrac{11}{4}\in \left( 2;3 \right)$.