Explanation:
a rectangle has two pairs of equally long sides and each of the 4 angles between the sides is 90 degrees.
given that ABCDEF is a regular hexagon, it means that all its sides are equally long, and therefore also all internal angles are the same size.
that means for ACDF that the sides DC and FA are equally long.
DEF and ABC are congruent triangles, as the sides DE = BC, EF = AB, and the angle E is congruent to angle B.
based on the SAS criteria this confirms that DF is congruent to AC.
the angles ACB, CAB, EDF and EFD have to be all the same, and therefore also the angles FDC, DCA, CAF and DFA.
so, all 4 angles of the parallelogram ACDF are the same, and the sum of all these angles has to be 360 degrees, we have
360 = 4×angle
angle = 90 degree.
so, all angles are 90 degrees and we have two pairs of equally long sides : this process that ACDF us a rectangle