a)
ĐKXD:
$\left \{ {{x\ge0} \atop {\sqrt{x}-2\ne0}} \right.$
`⇔`$\left \{ {{x\ge0} \atop {\sqrt{x}\ne2}} \right.$
`⇔`$\left \{ {{x\ge0} \atop {x\ne4}} \right.$
b)
ĐKXD:
$\left \{ {{x\ge0} \atop {\sqrt{x}+2\ne0}} \right.$
`⇔`$\left \{ {{x\ge0} \atop {\sqrt{x}\ne-2}} \right.$
`⇔x≥0`