What is "triangle"

In the plane, triangle (also accepted as trilateral) is the geometric figure that occupies the internal space bounded by three line segments that compete, two by two, at three different points forming three sides and three internal angles that add up to 180°.

The triangle is the only polygon that has no diagonals and each of its external angles is supplemental to the adjacent internal angle.

The perimeter of a triangle is the sum of the measures of its sides.

It is called the inner region of a triangle of convex region (curved on the outer face) and the outer region of concave region (curved on the inner face).