The cosine law

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Learn it c² = a² + b² − 2ab cos C. Use it to find a side when you know the other two sides and the angle between them (SAS), or to find an angle when you know all three sides (SSS).

Example: a = 8, b = 11, ∠C = 52°. Find c.

c² = 64 + 121 − 2(8)(11) cos 52° ≈ 76.6.
c ≈ 8.8.

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