Answer
$\sin\theta$
Work Step by Step
Use the subtraction formula for cosine, $\cos (A+B)=\cos A\cos B-\sin A\sin B$:
$\cos(270^\circ+\theta)$
$=\cos 270^\circ\cos \theta-\sin 270^\circ\sin \theta$
$=0*\cos\theta-(-1)*\sin\theta$
$=\sin\theta$