Question

If im given cos81 = 0.1564, how do i evaluate other cosines without a calculator? Like cos99 and cos279 for example.

Answer 1

As property of trigonometry:

cos x = - cos(180 - x)

=> cos 99 = - cos(180 - 99) = - cos 81 = - 0.1564

cos x = cos(360 - x)

=> cos 279 = cos(360 - 279) = cos 81 = 0.1564

Hope this helps!

:)