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∵長方形紙片ABCD沿AF折疊,使B點落在B′處,∴∠B′AF=∠BAF,∵AB′‖BD,∴∠B′AD=∠ADB=20°,∴∠B′AB=20°+90°=110°,∴∠BAF=110°÷2=55°.∴∠BAF應為55度時才能使AB′‖BD.
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