\[y = \frac{4 - 2x}{5};\ \ \ D(y) = R\]
\[x_{2} > x_{1};\ \ \ тогда\ \ x_{2} - x_{1} > 0\]
\[y\left( x_{2} \right) = \frac{4 - 2x_{2}}{5}\]
\[y\left( x_{1} \right) = \frac{4 - 2x_{1}}{5}\]
\[y\left( x_{2} \right) - y\left( x_{1} \right) =\]
\[= \frac{4 - 2x_{2}}{5} - \frac{4 - 2x_{1}}{5} =\]
\[= \frac{2 \cdot \left( x_{1} - x_{2} \right)}{5} < 0\]
\[Следовательно,\ \]
\[убывает \Longrightarrow ч.т.д.\]