The third criterion is the most “geometric” of all: the three sides are enough, without any information about the angles. It is the reason why a triangle, unlike a quadrilateral, is a rigid figure.
Theorem — Third congruence criterion
If two triangles have the three sides respectively congruent, then they are congruent.
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Topics: Euclidean geometry
Concepts: Congruence · Congruence criteria · Triangle
Skills: Synthetic geometry