如图四边形ABCD是梯形,AD平行BC,AD

Question

如图四边形ABCD是梯形,AD平行BC,AD

Answer 1

过点A作AE⊥BC于E,过点D作DF⊥BC于F,设BC＝2K
∵AB＝AC,AB⊥AC
∴等腰RT△ABC,AB＝AC＝BC/√2＝√2K
∵AE⊥BC
∴AE＝BC/2＝K
∵DF⊥BC
∴矩形AEFD
∴DF＝AE＝K
∵BD＝BC＝2K
∴∠CBD＝30
∴∠BCD＝（180-∠CBD）/2＝75
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