Answer: In two triangles, (1), if all three sides are equal, the two triangles are called congruence for short. (2) If two sides correspond, the angles between the two corresponding sides correspond; These two triangles are congruent. Edge for short. (3) If two more angles are equal in correspondence, if the sides of the two corner clips are equal in correspondence, or the opposite sides are equal in correspondence, then the two triangles are congruent; The former is called the corner; The latter is called corner edge for short. These are four ways to prove the congruence of two triangles. All triangles are proved by these four methods, and it is easier to prove isosceles triangles and equilateral triangles with special angles (right angles).

If the tangent of the circle is proved, 1 and the tangent diameter (or radius) of the circle are perpendicular to the tangent. 2. There is only one intersection point between tangent and circle. If there are two intersections, it is tangent; If there is no intersection, it is separation. These two methods are used to prove that a circle is tangent to a straight line.