Cosine Rule

🎬 Video Tutorial

(0:01) Cosine Rule: Use it to find missing sides or angles in non-right-angled triangles.
(0:13) Cosine Rule Formula: For any triangle, the formula is: $a^2 = b^2 + c^2 – 2bc \cdot \cos(A)$, where $A$ is the angle opposite side $a$.
(0:28) Finding a Missing Side: If two sides and the included angle are known, plug values into the Cosine Rule to solve for the third side.
(1:57) Finding an Angle with the Cosine Rule: Rearrange the formula to find the cosine of an angle when all three sides are known.

📂 Revision Cards

Cosine Rule formula a² = b² +c² -2bc·cos(A) explained with a triangle diagram labelled with sides a, b, c, and angle A. — 1) Cosine Rule

Using the cosine rule to find a side in a triangle with sides 3 cm and 5 cm and an angle of 60 degrees between them. — 2) Cosine Rule, Finding Sides

Triangle GEF with sides 4 cm, 3 cm, and 2 cm, applying the cosine rule to find angle F as 47 degrees using a calculator. — 3) Cosine Rule, Finding Angles

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