[tex]\cos\theta=\dfrac{\sqrt2}{2}\\\\\cos\dfrac{\pi}{4}=\dfrac{\sqrt2}{2}\ \text{and}\ \cos\left(-\dfrac{\pi}{4}\right)=\dfrac{\sqrt2}{2}\\\\\text{The period of cosine is}\ 2\pi.\ \text{Therefore your answer is}\\\\\theta=\dfrac{\pi}{4}+2k\pi\ \vee\ \theta=-\dfrac{\pi}{4}+2k\pi\ for\ k\in\mathbb{Z}[/tex]
