e., $\mathrm{MC}(x) = \frac{d}{dx} \mathrm{SC}(x)$. Using the given cost function, we have:
$$\mathrm{SC}(x) = 8,000 + 400x - 0.5x^2$$
$$\mathrm{MC}(x) = \frac{d}{dx} \mathrm{SC}(x) = 400 - x$$
Substituting $x=100$, we get:
$$\mathrm{MC}(100) = 400 - 100 = 300$$
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