Key points: (1) Trilateral relationship of triangle: the sum of any two sides is greater than the third side, and the difference between the two sides is less than the third side. (2) The sum of the interior angles of a triangle is 180 degrees and its proof idea. Concepts of the inside and outside angles of a triangle (3) Understanding and drawing of the midline, height line and bisector of each vertex of an arbitrary triangle. (4) congruent triangles property, which can infer the equal relationship among angles, line segments and areas. (5) Master the four judgment conditions of triangular congruence, and use them flexibly to judge triangular congruence. (6) Grasp the properties of the median perpendicular and bisector of a line segment, and use them to deduce that two line segments are equal, and use these two theorems to solve some problems in real life (7) Stability of triangles (8) Drawing of triangles that are all equal to known triangles, angular bisectors and median perpendicular lines.

Difficulties: the solution of the external angle of triangle, the solution of congruence condition, and the application of the theorem of the median vertical line and the bisector of angle of line segment. . . .

I hope these are useful to you.