It can be proved through many congruency axioms such as
1) A.S.A - angle side angle - if 2 angles which has a equal side in between it are equal in both triangles.
2) S.A.S - side angle side - if 2 sides which has a angle in between it are equal in both triangles
3) S.S.S - side side side - if all sides are equal
4) A.A.S - angle angle side - if 2 angles and a side is equal
5) R.H.S - right-angled hypotenuse side - if both triangle has a angle of 90 degree and equal hypotenuse and side