(c) $K_{\max} = \frac{1}{2}mv^{2}$ and $r_{0} = \frac{mv}{qb} \Rightarrow v = \frac{qBr_{0}}{m}$ $\Rightarrow K_{\max} = \frac{1}{2}m\left( \frac{qBr_{0}}{m} \right)^{2} = \frac{q^{2}B^{2}r_{0}^{2}}{2m}$
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Ph.D. in Physics with 19+ years of teaching experience.