Let $f(x) = \begin{cases} \cos x & \text{if } x \geq 0 \\ -\cos x & \text{if } x<0 \end{cases}$. Which one of the following statements is not true?
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When looking for discontinuities in piecewise functions, focus your attention on the points where the function definition changes. If the limits from both sides don't "meet," the function is not continuous there.