A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices A, B, and C is denoted. In Euclidean geometry, any three points, when non-collinear, determine a unique triangle and simultaneously, a unique plane (i.e. a two-dimensional Euclidean space). Two Triangles said to be congruent if they have the same shape and size, or if one has the same shape and size as the mirror image of the other. In other words, two triangles are congruent if their corresponding sides are equal in length, in which case their corresponding angles are equal in measure.
In a Euclidean space, the sum of angles of a triangle equals the straight angle (180 degrees, π radians, two right angles, or a half-turn). A triangle has three angles, one at each vertex, bounded by a pair of adjacent sides. The triangle inequality states that the sum of the lengths of any two sides of a triangle must be greater than or equal to the length of the third side.