\[\begin{array}{l}
C = 5\left| {1 - 4x} \right| - 1\\
Ta\,\,co:\,\,\,\left| {1 - 4x} \right| \ge 0\,\,\forall x\\
\Rightarrow 5\left| {1 - 4x} \right| \ge 0\,\,\forall x\\
\Rightarrow 5\left| {1 - 4x} \right| - 1 \ge - 1\,\forall x\\
Dau\,\, = \,\,xay\,\,ra \Leftrightarrow 1 - 4x = 0 \Leftrightarrow x = \frac{1}{4}\\
Vay\,\,Min\,\,C = - 1\,\,khi\,\,x = \frac{1}{4}.
\end{array}\]
