Answer:

It is proved that the sum of internal angles of triangle is 180.

(1) Extend BC to D (using the true proposition that "line segments can be extended")

(2) Make CE∨AB at point C .. (Use "a parallel line that passes through a point outside a straight line, which can be a known straight line")

(3)∠A=∠ 1 (using "two straight lines are parallel and the internal angles are equal")

(4)∠B=∠2 (using "two straight lines are parallel and the same angle is equal")

(5) ∠ 1+∠ 2+∠ ACB = 180 (using "the degree of a flat angle")

(6)∠A+∠B+∠ACB=∠ 1+∠2+∠C (use "equivalent substitution")

(7) ∠ A+∠ B+∠ ACB = 180 (using "equivalent substitution")

Extended data:

Attributes of triangle edges:

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.

The difference between two sides of a triangle is smaller than the third side.

Attribute of triangle angle:

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、? 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.