Question

Why is cos(pi) and cos (-pi) both equal to -1?

Answer 1

One of the reasons is because `cos` is an even function.

This means that `costheta=cos(-theta)`

An example is the present case `cospi=cos(-pi)=-1`

Answer 2

A full circle is `2pi`

`pi` is half way around the circle counter-clockwise.

`-pi` is half way around the circle clockwise.

`cos(pi) = cos(-pi)` are the both `cos` values for the same place

They are both equal to `(-1)`
because
if viewed as a unit circle centered on the Cartesian origin
the `cos` is the `x` value
and, at halfway around the unit circle,
`x=-1`

Answer 3

This is what a cosine graph looks like. Since `pi` in degrees is 180, it is exactly the same distance from 0 degrees.

Explanation:

graph{cos(x) [-4.76, 4.84, -2.64, 2.16]}