This means that the integral from $-\frac{\pi}{4}$ to $\frac{\pi}{4}$ is the same as twice the integral from $0$ to $\frac{\pi}{4}$. So, we can rewrite the integral as:
$$
\int_{-\pi / 4}^{\pi / 4} \cos x d x = 2 \int_{0}^{\pi / 4} \cos x d x
$$
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