As shown in the figure, the known points E and C are on the line BF, be = CF, ab ‖ De, ∠ ACB = ∠ F

As shown in the figure, the known points E and C are on the line BF, be = CF, ab ‖ De, ∠ ACB = ∠ F

It is proved that: ∵ ab ‖ De, ∵ B = ∵ def. ∵ be = CF, ∵ BC = EF. ∵ ACB = ∵ F, ∵ B = ≌ defbc = EF ≌ ACB = ≌ F, ≌ ABC ≌ def (ASA)