It is proved that AB = DC and AD = BC, so the quadrilateral ABCD is a parallelogram, so AD//BC.
In geometry, two straight lines that never intersect (and never overlap) on the same plane are called parallel lines.
Parallel lines are important concepts in axiomatic geometry. The parallel axiom of Euclidean geometry can be expressed as "only one straight line is parallel to the known straight line at a point outside the straight line". However, its negative form "there is no straight line parallel to the known straight line at a point outside the straight line" or "there are at least two straight lines parallel to the known straight line at a point outside the straight line" can be used as a substitute for the parallel axiom of Euclidean geometry, and non-Euclidean geometry independent of Euclidean geometry can be deduced.
If both lines are parallel to the third line, then the two lines are also parallel to each other. If a∨b, b∨c, then a ∨ c.
References:
Baidu encyclopedia-parallel lines