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. Inference: An outer angle of a triangle is greater than any inner angle that is not adjacent to it.
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.
8. The sum of squares of two right-angled sides of a right-angled triangle is equal to the square of the hypotenuse (Pythagorean theorem). Inverse Theorem of Pythagorean Theorem: If the lengths of three sides of triangle A, B and C satisfy a? +b？ =c？ Then this triangle is a right triangle.
9. The midline of the hypotenuse of a right triangle is equal to half of the hypotenuse.
10, the three bisectors of the triangle intersect at one point, the straight lines of the three high lines intersect at one point, and the three middle lines intersect at one point.
1 1, the sum of squares of the lengths of the three center lines of a triangle is equal to 3/4 of the sum of squares of the lengths of its three sides.
12, triangles with equal base heights have equal areas.
13. The area ratio of equilateral triangles is equal to their height ratio, and the area ratio of equilateral triangles is equal to their base ratio.
14. Any midline of a triangle divides this triangle into two triangles with equal areas.
15. The bisector of the top angle of the isosceles triangle is on the same straight line as the height on the bottom edge and the midline on the bottom edge (the three lines are one).