- The sum of angles in a triangle is 180°.
- The height is the perpendicular distance from a point to the opposite side.
- The centre of a circumscribed circle is located at the intersection of the axes of the edges.
- The axis of an edge is a perpendicular to the centre of the edge.
- The centre of an inscribed circle is located at the intersection of the axes of the angles.
- The axis of an angle divides the angle into two equal halves.
- A median is the point connecting line with the centre of the opposite side.
- Medians intersect at the centre that divides their length in a 2:1 ratio.

- P – perimeter
- A – area

- a,b,c – edges
- h
_{a}– height to edge a

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