In a plane, a polygon with three sides.

For measurement purposes, we establish a base *b*, and a height *h* relative to the base.

### Properties

- A planar triangle is a convex figure.
- The vertex opposite the side chosen as the base is sometimes called the apex of the triangle .
- A triangle has three hauteurs, three medians, three perpendicular bisectorand three angle bisectors.
- Altitudes:
The three altitudes determine the orthocenter of the triangle.

- Medians:
The three medians intersect at the centre of gravity (the centroid) of the triangle.

- Perpendicular bisectors:
The three perpendicular bisectors intersect at the centre of the circle circumscribed about the triangle.

- Angle bisectors:
The three angle bisectors intersect at the centre of the circle inscribed in the triangle.

- Altitudes:

### Formulas

- The formula to calculate the perimeter
*P*of a triangle with sides*a*,*b*and*c*is: \(P=a+b+c\). - The formula to calculate the area
*A*of a triangle with a base*b*and an altitude*h*is: \(A=\dfrac{b × h}{2}\). - Heron’s formula can be used to calculate the area
*A*of a triangle given the lengths of the three sides, where s is the semiperimeter of the triangle: \(A=\sqrt{s(s\space –\space a) (s\space –\space b) (s\space –\space c)}\).

### Examples