Consider the function

$f(x,y,z)=\{\begin{array}{ccc}\frac{2x+3y-z}{\sqrt[\phantom{2}]{x^2+y^2+z^2}},& (x,y,z)(0,0,& 0)\\ 1,& (x,y,z)=(0,0,& 0)\end{array}$

Is $f$ continuous at $(0,0,0)?$ What about the function