ngoai ciia goc : fiAfc, CXt), I5A6 theo thuT t u do.Do cac tam gidc can tai dinh A nen cacphan giac ngoai song song vdi canh day, nen :At, / / B C c (BCD)A t a Z / C D e (BCD);At3//BDc(BCD)At,, At2, At3 // (BCD)=> A t , , At2, Ata dong ph^ng (
trong (P) // (BCD) \k (P) qua A) (dpcm).
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