`a)` Ta có:

`\sqrt{x} >=0, AA x >=0`

`=>\sqrt{x}+42>=42, AA x>=0`

Hay `A>=42, AA x>=0`

Dấu "`=`" xảy ra

`⇔\sqrt{x}=0`

`⇔x=0`

Vậy Min`A=42 ⇔x=0`

`b)` Ta có:

`\sqrt{x+3} >=0, AA x >=-3`

`=>-12+\sqrt{x}>=-12, AA x>=-3`

Hay `B>=-12, AA x>=-3`

Dấu "`=`" xảy ra

`⇔\sqrt{x+3}=0`

`⇔x=-3`

Vậy Min`B=-12 ⇔x=-3`

`c)` Ta có:

`|x|>=0, AA x in RR`

`=> |x|-8>=-8, AA x in RR`

Hay `C>=-8, AA x in RR`

Dấu "`=`" xảy ra

`⇔|x|=0`

`⇔x=0`

Vậy Min`C=-8 ⇔x=0`