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)
