In the given quadrilateral ABCD, if AD = BC and ∠DAB = ∠CBA, then what is the relation between ∠ABD and ∠BAC ?
In ΔABD and ΔBAC,
AD = BC (Given)
∠BAD = ∠CBA (given)
AB = BA (side common to both triangles)
Hence, ΔABD≅ΔBAC
by SAS criterion for congruency of triangles.
Thus, ∠ABD = ∠BAC
(congruent parts of congruent triangles).