what is the definition of triangle in math
Triangle 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   △ A B C  {\displaystyle \triangle ABC} .
Perpendicular The legs of a right triangle are perpendicular to each other.
Heptagon In geometry, a heptagon is a seven-sided polygon or 7-gon.
Set (mathematics) A set is a well-defined collection of distinct objects. The objects that make up a set (also known as the elements or members of a set) can be anything: numbers, people, letters of the alphabet, other sets, and so on. Georg Cantor, the founder of set theory, gave the following definition of a set at the beginning of his Beiträge zur Begründung der transfiniten Mengenlehre:[1]