Đáp án:
9C 11C
Giải thích các bước giải:
\(\begin{array}{l}
C9:\\
{\sin ^2}a + {\cos ^2}a = 1\\
\to {\sin ^2}a + \dfrac{1}{{16}} = 1\\
\to {\sin ^2}a = \dfrac{{15}}{{16}}\\
\to {\cot ^2}a = \dfrac{{{{\cos }^2}a}}{{{{\sin }^2}a}}\\
= \dfrac{1}{{16}}:\dfrac{{15}}{{16}} = \dfrac{1}{{15}}\\
\to \cot a = \dfrac{1}{{\sqrt {15} }}\\
\to C\\
C11:\\
\cot a = \dfrac{3}{4} \to \dfrac{{\cos a}}{{\sin a}} = \dfrac{3}{4}\\
\to \cos a = \dfrac{3}{4}\sin a\\
{\sin ^2}a + {\cos ^2}a = 1\\
\to {\sin ^2}a + \dfrac{9}{{16}}{\sin ^2}a = 1\\
\to {\sin ^2}a = \dfrac{{16}}{{25}}\\
\to \sin a = 0,8\\
\to C
\end{array}\)
