As shown in the figure, Bo = OC, ab = DC, BF ‖ CE, and a, B, C, D are on the same line

As shown in the figure, Bo = OC, ab = DC, BF ‖ CE, and a, B, C, D are on the same line

It is proved that: ∵ BF ∥ CE, ∥ FBO = ≌ Eco, ∥ BFO = ∥ CEO, in △ BOF and △ Coe, ∥ BFO = ∥ CEO ∥ FBO = ∥ ecobo = Co, ∥ BOF ≌ COE (AAS) ∥ BF = CE, ∥ FBO = ∥ Eco, ∥ ABF = ∥ DCE, in △ ABF and △ DCE, ab = DC ≌ ABF = dcebf = CE ∥