## Precalculus (6th Edition) Blitzer

$-\frac{\sqrt 2}{2}$
Step 1. Write $225^\circ=180^\circ+45^\circ=\pi+\frac{\pi}{4}$ Step 2. Using the Addition Formula, we have $sin(225^\circ)=sin(\pi)cos(\frac{\pi}{4})+cos(\pi)sin(\frac{\pi}{4})=-\frac{\sqrt 2}{2}$