Mathematically, a congruence theorem of two triangles is proved: if the hypotenuse and a right-angled side of two right-angled triangles are equal respectively, then the two right-angled triangles are congruent (abbreviated as HL), where: H is the abbreviation of hypotenuse and L is the abbreviation of leg.

A proof method of the coincidence of two triangles

The hypotenuse of two right-angled triangles is equal to the right-angled side, and the two triangles are congruent.