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周長就是AB+AC=20 因為AB=AC,所以∠B=∠C,又因為DE平行AC,DF平行AB,那麼就有∠B=∠EDB,∠C=∠FDC,則EB=ED,FC=FD,四邊形周長等於AE+AF+DE+DF=AE+BE+AF+FC=AB+AC=20
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