Consider triangles ABC and DEF. Side a is congruent to side f. Angle A is congruent to angle F. Angle B is congruent to angle E. What postulate or theorem can be used to prove that the two triangles are congruent?

B = E, A = F

Hence C = D (Because A + B + C = D + E + F = 180 degrees)

A = F, a = f,

Hence triangles are congruent.

When 3 corresponding angles are equal, and a = f (Sides opposite to equal angles), triangles are congruent.