Real Cosine Function has Zeroes

Theorem
The cosine function has at least two distinct zeros that are additive inverses of each other.

That is:
 * $\exists \theta \in \R \setminus \set 0: \map \cos \theta = 0 \wedge \map \cos {-\theta} = 0$

Proof
$\cos x$ is positive everywhere on $\R$.

From Derivative of Cosine Function and Derivative of Sine Function:
 * $\map {\dfrac {\d^2} {\d x^2} } {\cos x} = \map {\dfrac \d {\d x} } {-\sin x} = -\cos x$

Thus $\map {\dfrac {\d^2} {\d x^2} } {\cos x} = -\cos x$ would always be negative.

Thus from Second Derivative of Concave Real Function is Non-Positive, $\cos x$ would be concave everywhere on $\R$.

But from Real Cosine Function is Bounded, $\cos x$ is bounded on $\R$.

By Differentiable Bounded Concave Real Function is Constant, $\cos x$ would then be a constant function.

This contradicts the fact that $\cos x$ is not a constant function.

Thus, by Proof by Contradiction, $\cos x$ cannot be positive everywhere on $\R$.

Therefore:
 * $\exists \phi \in \R: \cos \phi < 0$

From Cosine of Zero is One:
 * $\cos 0 = 1 > 0$.

Since the Cosine Function is Continuous, by the Intermediate Value Theorem:
 * $\exists \theta \in \R: \cos \theta = 0$

Because Cosine of Zero is One:
 * $\theta \ne 0$

Hence from Cosine Function is Even:
 * $\map \cos {-\theta} = 0$