(c) Resultant acceleration
$= \sqrt{\left( \begin{array}{r}
tangential \\
acceleration
\end{array} \right)^{2} + \left( \begin{array}{r}
centripetal \\
acceleration
\end{array} \right)^{2}}$
$= \sqrt{a^{2} + \left( \frac{v^{2}}{r} \right)^{2}} = \sqrt{\frac{v^{4}}{r^{2}} + a^{2}}$