Answer
$\dfrac{\sqrt2}{2}$
Work Step by Step
We know that $\sin$, $\csc$, $\tan$ are odd functions, which means $f(-\theta)=-f(\theta).$
We also know that $\cos$, $\sec$, are even functions, which means $f(-\theta)=f(\theta).$
Therefore,
$\cos{\left(-\frac{\pi}{4}\right)}=\cos{\left(\frac{\pi}{4}\right)}=\dfrac{\sqrt2}{2}$
