Cho trước $f \in C\left( {{\mathbb{R}^2}} \right)$. Đặt
\[\Phi \left( r \right) = \mathop {\sup }\limits_{\sqrt {x_1^2 + x_2^2} \leqslant r} \left| {f\left( {{x_1},{x_2}} \right)} \right|,r > 0,\Phi \left( 0 \right) = f\left( {0,0} \right).\]
Chứng minh rằng $\Phi \in C\left( {\left[ {0, + \infty } \right[} \right)$.
