trigonometry
Trigonometry relates to triangles and the relationship between the length of their sides and the size of their angles.
A right-angled triangle has three ratios between the length of its sides:
In simple terms, they divide one of the sides of a right-angled triangle by one of the other sides of a right-angled triangle to give a ratio between the two sides.
The sine rule and cosine rule can be used to discover the length of a side or the size of an angle in any triangle.