Cosine Rule
Cosine Rule
Secondary 3
\(1.\) The Cosine Rule can be used to find an unknown angle or length depending on the given conditions.
\(c^2 = a^2 + b^2 - 2ab \cos C\)
\(b^2 = a^2 + c^2 - 2ac \cos B\)
\(a^2 = b^2 + c^2 - 2bc \cos A\)
\(2.\) Conditions to use Cosine Rule:
• Non right-angled triangle
• When given 2 sides and 1 included angle
• When given 3 sides
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Answer:The Cosine Rule is \(a^2=b^2+c^2-2bc\cos A\) (and its cyclic equivalents). Use it in a non-right-angled triangle when two sides and the included angle, or all three sides, are known.
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