Answer
$c^2 = a^2 + b^2 - 2ab\cos C$.
Explanation
When $C = 90^\circ$, $\cos 90^\circ = 0$, reducing the law of cosines to the Pythagorean theorem $c^2 = a^2 + b^2$.
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State the law of cosines for $\triangle ABC$ to find side $c$ when sides $a, b$ and included angle $C$ are known.
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