# Sine and Cosine Rule

This section looks at the Sine Law and Cosine Law.

**The Sine Rule**

The Law of Sines (sine rule) is an important rule relating the sides and angles of **any** triangle (it doesn't *have* to be right-angled!):

If a, b and c are the lengths of the sides opposite the angles A, B and C in a triangle, then:

__ a __= __ b __= __ c __

sinA sinB sinC

If you wanted to find an angle, you can write this as:

__sinA__ = __sinB__ = __sinC__

a b c

**This video shows you how to use the Sine rule**

**The Cosine Rule**

This also works in any triangle:

c² = a² + b² - 2abcosC

which can also be written as:

a² = b² + c² - 2bccosA

**This video show you how to use the Cosine rule**

**The area of a triangle**

The area of any triangle is ½ absinC (using the above notation).

This formula is useful if you don't know the height of a triangle (since you need to know the height for ½ base × height).