A triangle is a plane closed figure having three sides as well as three angles i.e. three lines joined together at there ends constitutes a closed figure named triangle.It has three vertices and zero diagonals. The sum of all the interior angles is 180 degrees and its sum of the sum of the exterior angles is 360 degrees.There are many types of triangles namely:
1) Equilateral triangle -- Having all sides equal and all the three angles equal.
2) Scalene triangle -- Having all the three sides and three angles unequal.
3) Isosceles triangle -- Having any two sides equal and their two angles equal.
4) Right triangle -- Having one angle equal top 90 degrees and the rest two of any angles.This is the most important type of triangle for us because various theories are related to this triangle or depends on this triangle.Hence, the Pythagoras Theorem is based on this triangle and the origin of the Trigonometry is from this triangle and its application is in the chapter of Vector analysis of the physical quantities .
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Other theorems of triangles are sum of three interior angles is 180 degrees and sum of exterior angles is 360 degrees. there are also rules of congruences of triangle like sas criteria sss criteria asa criteria etc