Cosine Rule (Leaving Cert Mathematics): Flashcards

Two sides and the included angle

The angle between two sides

$a^2 = b^2 + c^2 - 2bc \cdot \cos A$

The angle opposite side $a$

All three lengths of the triangle

$\cos A = \frac{b^2 + c^2 - a^2}{2bc}$

Use inverse cosine ($\cos^{-1}$)

Square root both sides