A triangle is a polygon with three sides and three angles. The sum of the angles in a triangle always equals 180 degrees. Triangles can be classified by their side lengths or by their angles The different types of triangles by side lengths are:

- Equilateral triangle: All three sides are of equal length
- Isosceles triangle: Two sides are of equal length
- Scalene triangle: All three sides have different lengths

The different types of triangles by angles are:

- Acute triangle: All angles are less than 90 degrees
- Right triangle: One angle is exactly 90 degrees
- Obtuse triangle: One angle is greater than 90 degrees

Some other properties of triangle are:

- The longest side of a triangle is opposite the largest angle, and the shortest side is opposite the smallest angle.
- The Pythagorean Theorem can be used to find the length of the hypotenuse of a right triangle if the lengths of the other two sides are known.
- The area of a triangle can be calculated using the formula: (1/2) base * height.
- The sum of the length of any two sides of a triangle must always be greater than the length of the third side, or else the three given lengths cannot form a triangle.
- In terms of application, triangles are used in many fields such as geometry, trigonometry, engineering, physics, and computer graphics. learn more about School Management System.