Answer
Please see the work below.
Work Step by Step
We know that
$a_r=\frac{v^2}{r}$
$\frac{da_r}{dt}=\frac{1}{r}\frac{d}{dt}(v^2)$
$\frac{da_r}{dt}=\frac{1}{r}2v\frac{dv}{dt}=\frac{2va_t}{r}$
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