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 bottom and isosceles triangles with equal waist and bottom, that is, equilateral triangles); 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.

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Attributes of triangle:

1. On the plane, the sum of the interior angles of a triangle is equal to 180 (interior angle sum theorem).

2. On the plane, the sum of the outer angles of a triangle is equal to 360 (the theorem of the sum of outer angles).

3. On the plane, the outer angle of a triangle is equal to the sum of two non-adjacent inner angles.

4. There are at least two acute angles among the three internal angles of a triangle.

5. At least one angle in the triangle is greater than or equal to 60 degrees, and at least one angle is less than or equal to 60 degrees.

6. The sum of any two sides of a triangle is greater than the third side, and the difference between any two sides is less than the third side.

7. In a right triangle, if an angle is equal to 30 degrees, then the right side opposite to the 30-degree angle is half of the hypotenuse.