Sine and Cosine Formulae:
sin x = sin (180 - x)
e.g. sin 130 = sin (180 - 130) = sin 50

cos x = -cos (180 - x)

The Sine Rule:
This works in any triangle:

     a    =    b    =    c  
   sinA     sinB      sinC

alternatively, sinA = sinB = sinC
                      a        b       c

NOTE: the triangle is labelled as follows:

The Cosine Rule:
c² = a² + b² - 2abcosC
can also be written as:
a² = b² + c² - 2bccosA

This also works in any triangle.

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).

© Matthew Pinkney