Which equation correctly uses the law of cosines to solve for the length s?

The law of cosines is c² = a² + b² − 2ab cos(C), where C is the included angle. Here the sides around the 100° angle are 9 and 10, and the opposite side is s, so s² = 9² + 10² − 2(9)(10)cos(100°). That matches choice D.

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