`text{Đáp án + Giải thích các bước giải}`
`text{_____________________________________________________}`
`B=4-|5x-2|-|3y+12|`
`text{Ta có:}`
$\begin{cases} |5x-2|≥0∀x⇒-|5x-2|≤0∀x\\|3y+12|≥0∀y⇒-|3y+12|≤0∀y \end{cases}$
`=>-|5x-2|-|3y+12|≤0∀x ; y`
`=>4-|5x-2|-|3y+12|≤4`
`=>B≤4`
Dấu `=` xảy ra `⇔`$\begin{cases} |5x-2|=0\\|3y+12|=0 \end{cases}$
`⇔`$\begin{cases} x=2/5\\y=-4 \end{cases}$
Vậy `MAX_{B}=4⇔x=2/5 ; y=-4`
`text{_____________________________________________________}`
`C=(-1)/3+21/(8.|15x-21|+7)`
`text{Ta có:}`
`|15x-21|≥0∀x=>8.|15x-21|≥0∀x`
`=>8.|15x-21|+7≥7`
`=>21/(8.|15x-21|+7)≤21/7`
`=>(-1)/3+21/(8.|15x-21|+7)≤(-1)/3+21/7`
`=>C≤8/3`
Dấu `=` xảy ra `⇔|15x-21|=0⇔x=21/15=7/5`
Vậy `MAX_{C}=8/3⇔x=7/5`
`
