One way to find its value is
cos(-x)*cos(x) - sin(-x)*sin(x) = = 1.
(On the way I used cos(-x) = cos(x) and sin(-x) = -sin(x)).
The other way is to use the addition formula of trigonometry for cosines,
which is = .
When you use it with = -x and = x, you will get
cos(-x)*cos(x) - sin(-x)*sin(x) = cos((-x) + x) = cos(0) = 1.
Both ways lead to the same result.
