The Cosine Curve - cos 20, cos 85

[Guest]

Hi there I would like to know how to do the following questions without drawing a graph. What is the formula?

The question: Express each of the following in terms of the cosine of another angle between 0° and 180°

a) cos 20°
b) cos 85°

Thank you for any help!

[Guest]

cos 60 = cos 3*20
and we can use this formula:

[Guest]

I will continue thinking and I will write later.