Question

How to find the cosine of 81 in terms of sine?

Answer 1

Explanation:

The cosine of angle Ф is √(1 - sin²Ф), which is positive if our angle 81 degrees is in Quadrant I.

sin 81° = 0.9877, so (sin 81°)² = 0.9755, and

cos 81° = 1 - 0.9755 = 0.0245