Ta có: \(\left|2x+1\right|\ge0\Rightarrow5.\left|2x+1\right|\ge0\)
\(\Rightarrow5.\left|2x+1\right|\ge4\)
\(\Rightarrow GTNN=4\Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{9}{10}\\x=-\dfrac{1}{10}\end{matrix}\right.\)
Ta có:
|2x + 1| \(\ge\) 0 \(\forall\) x \(\in\) R
\(\Rightarrow\) 5|2x + 1| \(\ge\) 0
\(\Leftrightarrow\) 5|2x + 1| - 4 \(\ge\) -4
Do đó (5|2x + 1| - 4)max = -4 khi |2x + 1| = 0
\(\Leftrightarrow\) 2x + 1 = 0
\(\Leftrightarrow\) 2x = -1
\(\Leftrightarrow\) x = \(\dfrac{-1}{2}\)
Vậy (5|2x + 1| - 4)max = -4 khi x = \(\dfrac{-1}{2}\)
