For every number #a# in #(0,oo)#, we have #lim_(xrarra)sqrtx = sqrta#. That is the definition of continuous at #a#.
At #0#, we have #lim_(xrarr0^+) sqrtx = sqrt0# which is the definition of continuous from the right at #0#.
So #sqrtx# is continuous on #[0, oo)#.
#lim_(xrarra)sqrtx = sqrta# can be proven using the definition of limit.