Hour of Math · SS2 · Unit SSGM9

The Cosine Rule for Angles — and the "Which Rule?" Strategy

Rearrange the cosine rule to $\cos A = \frac{b^2 + c^2 - a^2}{2bc}$ to find an angle from three sides (SSS), read a negative cosine as an obtuse angle, and apply a decision framework to choose between the sine and cosine rules.

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