Usually, Trigonometry is used to deal with right-angled triangles. Trigonometry helps to find out the sides and angles using the relationship between them. Both the Sine and Cosine rules apply to any triangle - not just right-angled triangles.


Labelling[edit | edit source]

A labled triangle

  • In right angled triangles, the side next to the angle you are using is called the 'adjacent', the side opposite, is called that. The longest side is called the hypotenuse.
  • For the sine rule, the cosine rule and areas of triangles, the sides are labelled a,b and c and the angles opposite a,b and c are labelled A, B and C.

Sine[edit | edit source]

Cosine[edit | edit source]

Tangent[edit | edit source]

Memory Aids[edit | edit source]

The sine, cosine, and tangents can be remembered by this rhyme: Some old hag came and had tea one afternoon (sine: opposite/hypotenuse, cosine: adjacent/hypotenuse, tangent: opposite/adjacent).

Another good memory aid is SOH, CAH, TOA. They are abbreviations for [sine=opposite/hypotenuse], [cos=adjacent/hypotenuse] and [tan=opposite/adjacent].

The Sine Rule[edit | edit source]

Sine equation.jpg

The Cosine Rule[edit | edit source]

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