$\\$
`a,`
`C=5 - |x+3|`
Vì `|x+3| ≥0∀x -> - |x+3| ≤0∀x`
`-> 5 -|x+3| ≤5∀x`
`->C≤5∀x`
Dấu "`=`" xảy ra khi :
`↔ |x+3|=0`
`↔x+3=0`
`↔x=-3`
Vậy `max C=5 ↔x=-3`
$\\$
`b,`
`D=- |x-3/7| - 1/4`
Vì `|x-3/7| ≥0∀x -> - |x-3/7| ≤0∀x`
`-> - |x-3/7| -1/4 ≤ (-1)/4 ∀x`
`->D ≤ (-1)/4 ∀x`
Dấu "`=`" xảy ra khi :
`↔ |x-3/7|=0`
`↔x-3/7=0`
`↔x=3/7`
Vậy `max D=(-1)/4 ↔x=3/7`