In the parallelogram ABCD, take ad and BC as sides, make positive △ ade and positive △ BFC outward respectively, connect dB and EF at point O, and prove that the quadrilateral debf is a parallelogram

The parallelogram ABCD takes AD and BC as sides and makes positive △ ade and positive △ BFC outward respectively
So BF = de angle ead = angle FCB = 60 ° angle bad = angle DCB, namely angle EAB = angle DCF
So Ba = DC, AE = ad = BC = CF
So △ EAB is equal to △ FCD
So be = DF
Because de = BF be = DF, the quadrilateral debf is a parallelogram

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