A triangle has three sides and three angles;

The sum of any two sides of a triangle is greater than the third side (equivalent: the difference between any two sides is less than the third side);

The sum of the internal angles of the triangle is189;

The outer angle of one angle of a triangle is equal to the sum of two inner angles that are not adjacent to it;

Triangles have structural stability.

Triangle is divided into:

Triangle according to the size of the angle can be divided into:

Obtuse triangle: one angle is greater than 90.

Right triangle: There is an angle equal to 90. Using Pythagorean theorem, it can be found that the sum of squares of three sides, that is, two right angles, is equal to the square of the third side.

Acute triangle: all three angles are less than 90.

Triangle according to the size of the edge can be divided into:

Equilateral triangle: A triangle with three equal sides is called an equilateral triangle. Three sides of an equilateral triangle are equal, and three angles are equal to 60.

Isosceles triangle: A triangle with two equal sides is called an isosceles triangle. The two base angles of an isosceles triangle are equal, that is, equilateral and equiangular.

Ordinary triangle: a triangle with three sides and three angles that are not equal.