Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$\dfrac{13\pi}{180} \ rad$
In order to convert the value from degrees to radians, we multiply by $\dfrac{π}{180^{\circ}}$. Therefore $13^{\circ}=(13) \times \dfrac{\pi}{180}\text{radians} \\=\dfrac{13\pi}{180} \ rad$