Cho 2 dtron(O1;r1) va` dtron (O2;r2) ngoa`i nhau,R1 #R2.mOT DTRON (O) thay doi tiep xuc ngoa`i vs (O1) tai A va` (O2) ta.i B.cm rang:Dthang AB luon di qua 1 diem co dinh.