Internal angle, in mathematical terms, the angle formed by two adjacent sides of a polygon is called the internal angle of a polygon. Mathematically, the sum of the internal angles of a triangle is 180, and the sum of the internal angles of a quadrilateral (polygon) is 360. And so on, add an edge, and the sum of internal angles will add 180.

The formula of the sum of internal angles is (n-2) × 180. The degree of each internal angle of a regular polygon is: (n-2) × 180 ÷ n For example, the sum of the internal angles of a triangle is the sum of three angles in a triangle, and an internal angle is any one of them.

Triangle is a closed figure composed of three line segments on the same plane but not on the same straight line, which has applications in mathematics and architecture. Ordinary triangles are divided into ordinary triangles (three sides are unequal) and isosceles triangles (isosceles triangles with unequal waist and equal waist and bottom).

According to the angle, there are right triangle, acute triangle and obtuse triangle, among which acute triangle and obtuse triangle are collectively called oblique triangle. A closed figure composed of three line segments that are not on the same straight line is called a triangle. A figure surrounded by three straight lines on a plane or three arcs on a sphere is called a plane triangle.

Judge:

1. If three sides of a triangle are proportional to three sides of another triangle, then the two triangles are similar (abbreviated as: two triangles with three sides corresponding to a ratio are similar).

2. If two sides of a triangle are proportional to two sides of another triangle and the included angles are equal, then the two triangles are similar (abbreviated as: triangles with two proportional sides and equal included angles are similar).

3. If two angles of a triangle correspond to two angles of another triangle, then the two triangles are similar (referred to as two triangles with equal angles are similar for short).

4. If the hypotenuse and right-angled side of a right-angled triangle are proportional to the hypotenuse and right-angled side of another right-angled triangle, then the two triangles are similar.