Firstly, we can understand the symmetry of points, which can be directly understood as the distance between points M and N2 and lines A and B is equal, and secondly, the line M..n is the symmetry axis to divide the straight line where AB is located into two identical parts.

It can be concluded that MC=CN and MN is perpendicular to AB. Knowing from the topic that AC=BC, we can pass two delta congruence proofs from (corner edge).