A.\( - \dfrac{{\sqrt 3 }}{3}\)
B.\(\dfrac{{\sqrt 3 }}{3}\)
C.\( - \sqrt 3 \)
D.\(\sqrt 3 \)

A.\(\dfrac{{\sqrt 3 }}{2}\)
B.\(1\)
C.\( - 1\)
D.\( - \dfrac{{\sqrt 3 }}{2}\)

A.\(8\cos 20^\circ \)
B.\(4\cos 20^\circ \)
C.\(4\sin 20^\circ \)
D.\(8\sin 20^\circ \)

A.\(\dfrac{{48 + 25\sqrt 3 }}{{11}}\)
B.\(\dfrac{{8 - 5\sqrt 3 }}{{11}}\)
C.\(\dfrac{{8 - \sqrt 3 }}{{11}}\)
D.\(\dfrac{{48 - 25\sqrt 3 }}{{11}}\)

A.\(\dfrac{\pi }{3}\)
B.\(\dfrac{\pi }{4}\)
C.\(\dfrac{\pi }{6}\)
D.\(\dfrac{\pi }{2}\)

A.\(\dfrac{5}{4}\)
B.\(\dfrac{3}{4}\)
C.\(\dfrac{5}{8}\)
D.\(\dfrac{3}{8}\)

A.\(\dfrac{1}{2}{\sin ^2}a\)
B.\( - \dfrac{1}{2}{\cos ^2}a\)
C.\( - \dfrac{1}{2}{\sin ^2}a\)
D.\(\dfrac{1}{2}{\cos ^2}a\)

A.\(0\)
B.\(1\)
C.\(2\)
D.\(3\)

A.\( - \sin \alpha \)
B.\( - \cos \alpha \)
C.\(\cos \alpha \)
D.\(\sin \alpha \)

A.\(m < \dfrac{{1 - \sqrt {21} }}{2}\)
B.\(m < \dfrac{{1 - \sqrt {21} }}{2}\) hoặc \(m > \dfrac{{1 + \sqrt {21} }}{2}\)
C.\(m > \dfrac{{1 + \sqrt {21} }}{2}\)
D.\(\dfrac{{1 - \sqrt {21} }}{2} < m < \dfrac{{1 + \sqrt {21} }}{2}\)