AD || BC • AB = CD • ∠BAD = ∠CDA (a given condition ∵ ABCD is an isosceles trapezium)
The diagonals AC and BD create two ∆'s: ∆ABD, and ∆ACD
The two ∆'s are congruent (SAS).
(S): AB = CD (A): ∠BAD = ∠CDA (S): AD = DA (common side) The corresponding sides of the two ∆'s, BD and AC, are also congruent. (the two diagonals of the trapezium/trapezoid)
