Answer
$\dfrac{\sqrt{2}}{2}$
Work Step by Step
The cosine is an even function so
$\cos{(- \theta)} = \cos{\theta}$
Thus,
$\cos{\left(- \dfrac{\pi}{4} \right) } =\cos{\left( \dfrac{\pi}{4} \right) } = \boxed{\dfrac{\sqrt{2}}{2}}$
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