r = 10cm
[tex]\[\begin{array}{l}A = \pi {r^2}\\\\\frac{{\partial A}}{{\partial r}} = \pi (2)(r)\\\\\frac{{\partial A}}{{\partial r}} = \pi (2)(10)\\\\\frac{{\partial A}}{{\partial r}} = 20\pi cm\\\\\frac{{\partial A}}{{\partial r}} = 62.8cm\end{array}\][/tex]