1. Definition of triangle: A triangle is a figure composed of three line segments, in which the sum of the lengths of any two line segments is greater than the length of the third line segment.
2. Classification of triangles: According to the length relationship of three sides, triangles can be divided into equilateral triangles, isosceles triangles and general triangles. According to the relationship between angles, triangles can be divided into acute triangles, right triangles and obtuse triangles.
3. The nature of triangle: the sum of the internal angles of triangle is 180 degrees, that is, the sum of the degrees of three internal angles is 180 degrees. The outer angle of a triangle is equal to the sum of its non-adjacent inner angles. Three internal angles of an equilateral triangle are all 60 degrees, two internal angles of an isosceles triangle are equal, and one internal angle of a right triangle is 90 degrees.
4. Area formula of triangle: the area of triangle is equal to the length of base multiplied by half height, that is, S= 1/2bh. Where b is the length of the bottom and h is the length of the height.
5. Pythagorean theorem of triangle: In a right triangle, the square on the right is equal to the sum of the squares of the other two sides, that is, a? +b？ =c？ . Where a and b are right angles and c is hypotenuse.
6. Triangle Cosine Theorem: For any triangle ABC, let A, B and C be the length of three sides, and A, B and C are the corresponding internal angles, then there is C? =a？ +b？ -2abcosC .
The above is the basic knowledge about triangles. Mastering this knowledge can help us better understand and apply triangles.