Answer
$-\frac{5\pi}{4}$ rad.
Work Step by Step
Using the fact that $\pi \: \textrm{rad.} = 180^\circ$, $$-225^\circ \cdot \frac{\pi \: \textrm{rad.}}{180^\circ}=-\frac{225\pi}{180}=-\frac{5\pi}{4}$$
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