A geometry problem by Chris San

$\cos(\theta)$ is an even function which means $\color{#D61F06}{cos(-\theta)=\cos(\theta)}$ .

$\huge \color{#3D99F6}{\text{Simple Proof :-}}$

$\cos(-\theta)\,=\,\cos(0-\theta)\,=\, \underbrace{\cos(0)}_\text{1} \cdot \cos(\theta)\,+\,\underbrace{\sin(0)}_\text{0} \cdot \sin(\theta) \,\,..........\,\,\color{#624F41}{\cos(\alpha-\beta)=\cos(\alpha) \cdot \cos(\beta)\,+\, \sin(\alpha) \cdot \sin(\beta) }$ Hence, $\color{#20A900}{\cos(-\theta)\,=\,\cos(\theta)}$