When do I use the cosine rule?

When the sine rule does not apply: three known sides, or two sides with the angle between them.

Trigonometry Formulas — Examples and Uses

The trigonometry identities and rules needed for triangles and equations.

$\sin\theta = \frac{o}{h},\; \cos\theta = \frac{a}{h},\; \tan\theta = \frac{o}{a}$

Only valid in a right-angled triangle.

$\sin^2\theta + \cos^2\theta = 1$

True for every angle.

$\frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C}$

Use with two angles and a side, or two sides and a non-included angle.

$c^2 = a^2 + b^2 - 2ab\cos C$

Use with three sides, or two sides and the included angle.

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When do I use the cosine rule?: When the sine rule does not apply: three known sides, or two sides with the angle between them.