Đáp án:
$\begin{array}{l}
1)\sin a = - \dfrac{3}{4}\\
Do:\dfrac{{3\pi }}{2} < a < 2\pi \\
\Leftrightarrow \cos a > 0\\
\Leftrightarrow \cos a = \sqrt {1 - {{\sin }^2}a} = \sqrt {1 - \dfrac{9}{{16}}} = \dfrac{{\sqrt 7 }}{4}\\
a)\cos 2a = 1 - 2{\sin ^2}a = 1 - 2.\dfrac{9}{{16}} = - \dfrac{1}{8}\\
b)\sin 2a = 2\sin a.\cos a = 2.\dfrac{{ - 3}}{4}.\dfrac{{\sqrt 7 }}{4} = \dfrac{{ - 3\sqrt 7 }}{8}
\end{array}$
