Giải thích các bước giải:
Đặt $\tan \dfrac{x}{2}$=$t$
$\rightarrow t$=$-2$
Ta có : $\sin x$=$\dfrac{2t}{1+t^2}$=$\dfrac{-4}{1+4}$=$\dfrac{-4}{5}$
$\cos x$=$\dfrac{1-t^2}{1+t^2}$=$\dfrac{1-4}{1+4}$=$\dfrac{-3}{5}$
$\tan x$=$\dfrac{\sin x}{\cos x}$=$\dfrac{4}{3}$
$\tan x .\cot x$=1
$\Rightarrow \cot x$=$\dfrac{3}{4}$
$\Rightarrow A$=$\dfrac{3.\dfrac{-4}{5}+4.\dfrac{-3}{5}}{4.\dfrac{3}{4}+3.\dfrac{4}{3}}$=$\dfrac{-24}{35}$
