The sum of (1) triangles is equal to180;

(2) An outer angle of a triangle is equal to the sum of two inner angles that are not adjacent to it;

(3) The outer angle of a triangle is larger than any inner angle that is not adjacent to it;

(4) The sum of two sides of the triangle is greater than the third side, and the difference between the two sides is less than the third side;

(5) In the same triangle, the big side faces the big corner, and the big corner faces the big side.

Four special line segments in isosceles right triangle: angle bisector, midline, height and midline.

The intersection of the bisectors of a (1) triangle is called the center of the triangle. It is the center of the inscribed circle of a triangle, and its distance to each side is equal.

The center of the circumscribed circle of a triangle, that is, the outer center, is the intersection of the perpendicular lines of the three sides of the triangle, and its distances to the three vertices are equal.

(2) The intersection of the three center lines of a triangle is called the center of gravity of the triangle, and its distance to each vertex is equal to twice its distance to the midpoint of the opposite side.

Two right-angled triangles with unequal right-angled sides of extended data can be spelled as follows:

1, two isosceles triangles with different waist lengths;

2. Two parallelograms with different side lengths;

3. congruent triangles with three equal vertex angles;

4. Rectangular;

5. It is a triangle when it overlaps.