Plot3D[ 10 - Sqrt[100-x^2-y^2], {x,-11,11},{y,-11,11}]

Plot3D[ x^2+y^2, {x,-2,2}, {y,-2,2}]

Plot3D[ {10-Sqrt[100-x^2-y^2], x^2+y^2}, {x,-3,3},{y,-3,3}]