# Concise Selina Solutions for Class 9 maths Chapter 9 Triangles (Congruency in Triangles)

SSS stands for "side, side, side" and means that we have two triangles with all three sides equal. If three sides of one triangle are equal to three sides of another triangle, the triangles are congruent. The definition of a triangle is a shape with three angles and three sides. An example of something in the shape of a triangle is a piece of pizza. A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. 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). There are three types of a triangle based on the length of the sides: equilateral, isosceles, and scalene. Their angles can also classify triangles. In an acute triangle, all three angles are acute (less than 90 degrees). A right triangle contains one right angle and two acute angles. And an obtuse triangle contains one obtuse angle (greater than 90 degrees) and two acute angles.

Usually, we reserve congruence for two-dimensional figures, but three-dimensional figures, like our chess pieces, can be congruent, too. Think of all the pawns on a chessboard. They are all congruent. To summarize, congruent figures are identical in size and shape; the side lengths and angles are the same.