Question

Provet that? : `cos(-x) -= cosx`

Answer 1

This is a proof

Explanation:

We use the sum of angles formula:
`cos(A+-B) -= cosAcosB+-sinAsinB`

Put `A=0` and `B=-x` to get
`cos(0-x) -= cos0cosx-sin0sinx`

And we know that `sin0=0` and `cos0=1` so:
`cos(-x) -= (1)cosx - 0`
`:. cos(-x) -= cosx` QED