Đáp án:
$\begin{array}{l}
f'\left( x \right) + 2f\left( x \right) = 0\\
\Rightarrow \dfrac{{f'\left( x \right)}}{{f\left( x \right)}} = - 2\left( {do:f\left( x \right) > 0} \right)\\
\Rightarrow \int {\dfrac{{f'\left( x \right)}}{{f\left( x \right)}}dx} = \int { - 2dx} \\
\Rightarrow \int {\dfrac{1}{{f\left( x \right)}}.d\left( {f\left( x \right)} \right) = - 2x + C} \\
\Rightarrow \ln \left( {f\left( x \right)} \right) = - 2x + C\\
Cho:f\left( 1 \right) = ?\\
\Rightarrow f\left( x \right) = \\
\Rightarrow f\left( { - 1} \right) =
\end{array}$
