Answer: cos 810 = 0
Explanation:
Keep Subtracting 360 until you get your reference angle. You can also look at the image and know that it will be 90°
810 -360 = 450
450 - 360 = 90
At 90°, use unit circle, your x is cos and your y is sin
cos 90 = 0
cos 810 = 0