Take a vertex angle as the height, a parallelogram as the diagonal, and the sum of the congruence and the internal angles of the quadrilateral as the parallel lines of the opposite sides of an angle, and transfer the sum of the three internal angles to the same straight line by using the equality of the internal angles.

Draw a parallelogram at will and divide it into two triangles, which are congruent. Then the sum of two adjacent angles of parallelogram is 180, and we can find that the sum of three angles is 180, in which two angles are internal angles of a triangle, and the other angle can also be replaced by the internal angle of this triangle through parallel lines, thus proving.

Basic definition

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; A figure surrounded by three arcs is called a spherical triangle, also known as a triangle.

A closed geometry consisting of three line segments connected end to end is called a triangle. Triangle is the basic figure of geometric figure.