Let us first define Hypotenuse Leg (HL) congruence theorem:
If the hypotenuse and one leg of a right angle are congruent to the hypotenuse and one leg of the another triangle, then the triangles are congruent.
Given ACB and DFE are right triangles.
To prove ΔACB ≅ ΔDFE:
In ΔACB and ΔDFE,
AC ≅ DF (one side)
∠ACB ≅ ∠DFE (right angles)
AB ≅ DE (hypotenuse)
∴ ΔACB ≅ ΔDFE by HL theorem.
