Answer
$-\frac{3\pi}{2}$ rad.
Work Step by Step
Use the conversion $360^\circ=2\pi \, \textrm{rad.}$ to simplify the expression to $$-270^\circ \cdot \frac{2\pi \, \textrm{rad.}}{360^\circ}=-\frac{3\pi}{2} \, \textrm{rad.}$$
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