12a) A=| 2x - $\frac{1}{3}$| - 1$\frac{3}{4}$
ta có | 2x - $\frac{1}{3}$| $\geq$ 0
$\Rightarrow$| 2x - $\frac{1}{3}$| - 1$\frac{3}{4}$ $\geq$ - 1$\frac{3}{4}$ = $\frac{-7}{4}$
Dấu = xảy ra $\Leftrightarrow$ 2x - $\frac{1}{3}$ = 0
$\Leftrightarrow$ x = $\frac{1}{6}$
Vậy Min A = $\frac{-7}{6}$ $\Leftrightarrow$ x = $\frac{1}{6}$
b)B= $\frac{1}{3}$|x-2| + 2| 3 - $\frac{1}{2}$y| + 4
có | x-2| $\geq$ 0 |;|3 - $\frac{1}{2}$y |$\geq$ 0
$\Rightarrow$ $\frac{1}{3}$|x-2| + 2| 3 - $\frac{1}{2}$y| $\geq$ 0
$\Rightarrow$ $\frac{1}{3}$|x-2| + 2| 3 - $\frac{1}{2}$y| +4 $\geq$ 4
Dấu = xảy ra $\Leftrightarrow$ $\left \{ {{x-2 = 0} \atop {3-\frac{1}{2}y=0}} \right.$
$\Leftrightarrow$ $\left \{ {{x=2} \atop {y=6}} \right.$
Vậy Min B= 4 $\Leftrightarrow$ $\left \{ {{x=2} \atop {y=6}} \right.$
13a)A=2,25 - $\frac{1}{4}$|1+2x|
có $\frac{1}{4}$|1+2x| $\geq$ 0
$\Rightarrow$ 2,25 - $\frac{1}{4}$|1+2x| $\leq$ 2,25
Dấu = xảy ra $\Leftrightarrow$ 1+2x =0 $\Leftrightarrow$ x= $\frac{-1}{2}$
Vậy Max A = 2,25 $\Leftrightarrow$ x = $\frac{-1}{2}$
b)B=$\frac{1}{3+\frac{1}{2}|2x-3|}$
có $\Rightarrow$ 3 + $\frac{1}{2}$|2x-3|$\geq$ 3
$\Rightarrow$ $\frac{1}{3+\frac{1}{2}|2x-3|}$ $\leq$ $\frac{1}{3}$
Dấu = xảy ra $\Leftrightarrow$ 2x-3 = 0 $\Leftrightarrow$ x= $\frac{3}{2}$
Vậy MaxB=$\frac{1}{3}$ $\Leftrightarrow$ x = $\frac{3}{2}$
