ABSTRACT
M3.1. Plane Geometry M3.1.1. Triangles ◮ Plane triangle and its properties. 1◦. A plane triangle, or simply a triangle, is a plane figure bounded by three straight line segments (sides) connecting three noncollinear points (vertices) (Fig. M3.1a). The smaller angle between the two rays issuing from a vertex and passing through the other two vertices is called an (interior) angle of the triangle. The angle adjacent to an interior angle is called an external angle of the triangle. An external angle is equal to the sum of the two interior angles to which it is not adjacent.
