Question

What is the value of `cos( -pi)`?

Answer 1

`cos(-pi)=-1`

Explanation:

Consider a definition of a function `cos(theta)` on a unit circle as an abscissa (X-coordinate) of a point that lies on this unit circle and an angle from the positive direction of the X-axis to a radius to this point counterclockwise equals to `theta`.

On this unit circle a point that corresponds to `theta=-pi` has coordinates `(-1,0)` - this is th left most point on a circle.

Therfore, according to definition above, its abscissa is `-1`, which is the value of `cos(-pi)`.