The Cosine Rule (Leaving Cert Mathematics): Flashcards

Solving triangles when the Sine Rule cannot be used

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

$b^2 = c^2 + a^2 - 2ca \cos B$

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

Two sides and the included angle are given

All three sides are given

The angle between two known sides

Try Sine Rule first, use Cosine Rule when Sine Rule fails

Angle between 90° and 180° (obtuse angle)

Degree mode

All three sides and one angle in a triangle