Proof: ∫∠b+∠C = 180.

∴∠B and∠ C are complementary.

∴AB∥CD

AB = CD

∴ Quadrilateral ABCD is a parallelogram row (a group of parallel and equal quadrilateral rows with opposite sides are parallelogram rows).

4. It is proved that ∵ quadrilateral ABCD is a parallelogram, ABCD, AD∨CB,

∴ ∠ABD=∠CDB

∫EB and FD divide∠ ∠ABD and∠ ∠CDB equally, respectively.

∴∠FDB=∠EBD

∴∠B and∠ C are complementary.

∴DF∥EB

∫AD∨CB, E and F are on AD and CD respectively.

∴ Four Sides DEBF is a parallel four-sided line.