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.\(\dfrac{{1 - \sqrt {21} }}{2} < m < \dfrac{{1 + \sqrt {21} }}{2}\)

A.2028
B.2020
C.2019
D.2018

A.\(\sin \left( {x + y} \right) = \sin x + \sin y\)
B.\(\sin \left( {x + y} \right) = \sin x.\cos y + \cos x.\sin y\)
C.\(\cos \left( {x - y} \right) = \sin x.\cos y - \cos x.\sin y\)
D.\(\sin \left( {x - y} \right) = \cos x.\cos y - \sin x.\sin y\)

A.\(\cos \left( {x + y} \right) = \cos x.\cos y - \sin x.\sin y\)
B.\(\cos \left( {x + y} \right) = \cos x.\cos y + \sin x.\sin y\)
C.\(\cos \left( {x + y} \right) = \cos x.\sin y + \sin x.\cos y\)
D.\(\cos \left( {x + y} \right) = \cos x.\sin y - \sin x.\cos y\)

A.\(\sin \left( {a + \dfrac{\pi }{6}} \right) = \sin a + \dfrac{1}{2}\)
B.\(\sin \left( {a + \dfrac{\pi }{6}} \right) = \dfrac{1}{2}\sin a - \dfrac{{\sqrt 3 }}{2}\cos a\)
C.\(\sin \left( {a + \dfrac{\pi }{6}} \right) = \dfrac{{\sqrt 3 }}{2}\sin a\, - \dfrac{1}{2}\cos a\)
D.\(\sin \left( {a + \dfrac{\pi }{6}} \right) = \dfrac{{\sqrt 3 }}{2}\sin a + \dfrac{1}{2}\cos a\)

A.\( - \sqrt 3 \)
B.\( - 1\)
C.\(1\)
D.\(\sqrt 3 \)

A.\(\sin {15^ \circ } = \dfrac{{\sqrt 6  + \sqrt 2 }}{4}\)
B.\(\cos {15^ \circ } = \dfrac{{\sqrt 6  - \sqrt 2 }}{4}\)
C.\(\cos {75^ \circ } = \dfrac{{\sqrt 6  + \sqrt 2 }}{4}\)
D.\(\cos {75^ \circ } = \dfrac{{\sqrt 6  - \sqrt 2 }}{4}\)