From DT Online
Trigonometric ratios are used to calculate angles and lengths of sides of triangles. Most commonly, the ratios relate to a right-angled triangle and are known as the Sine, Cosine, or Tangent of a given angle.
Trigonometry also exists for non-right-angled triangles but since a non-right-angle triangle can be bisected to create two right-angle triangles, most problems can be reduced to calculations on right-angle triangles.
An exception is the Sine Rule shown opposite which can be very helpful when determining the unknown forces in a Force Triangle for example.