Đáp án:
$\begin{array}{l}
a)tanx = \dfrac{{\sin x}}{{\cos x}};cotx = \dfrac{1}{{\tan x}} = 3\\
Do:\dfrac{1}{{{{\cos }^2}x}} = {\tan ^2}x + 1 = \dfrac{4}{3}\\
\Rightarrow {\cos ^2}x = \dfrac{3}{4}\\
\Rightarrow \cos x = \dfrac{{\sqrt 3 }}{2}\\
\Rightarrow \sin x = \tan x.\cos x = \dfrac{{\sqrt 3 }}{6}\\
b)\cot x = \dfrac{3}{4}\\
\Rightarrow \tan x = \dfrac{4}{3} = \dfrac{{\sin x}}{{\cos x}}\\
\Rightarrow \dfrac{1}{{{{\cos }^2}x}} = \dfrac{{25}}{9}\\
\Rightarrow \cos x = \dfrac{3}{5}\\
\Rightarrow \sin x = \dfrac{4}{5}
\end{array}$
