You can prove triangles’ congruence with things called axioms (or postulates). For triangles, there are four:
(S means Side, A means Angle)
SAS
SSS
ASA
AAS
If you go round a triangle, in order (so aside, then the next angle, next side, etc.), and find that everything from one of these postulates is equal, you have two congruent triangles.
(These triangles’ sides and angles, which are marked on the image, are equal to the other’s)
This diagram shows the SAS axiom, as the equal parts of the triangle go in order of a side (Y to Z), an angle (Y to Z to X), and another side (Z to X).
Hope this helps!
