. Ndu BN =CM thi frBC-'NC b-
nrsthnrs uh c0llo'ffit;tl €ttsnrt€Itt Khi mOt ph6p to6n dd duo c xdc dinh tr0n mOt
Khi mOt ph6p to6n dd duo. c xdc dinh tr0n mOt qp hqp tft ngu& ta ggi "nghich dio cira mpt pJtin
tff' tehgt phdn ttr mi ndu c6 thi kdt qry cqa ph6p to6n gifra nO ,a pndn trl d5 cho lI phdn ttr trung
hba.
Vi du trong tAp sd hfru ti Q, vor ph6p to6n nhdn,
! la nshich d6o cua 3 vi 3 x I = r lh phdn tir
3 '* "o^---- --- 3
trung hba.
C[ng vdi nguycn li Ay ta c6 nghlch dio cira m6t *u tr#, cira"mot ph6p-bieh Adi (phdp toiln lh tich
nhfrng 6nh xa). Nhung nguor ta cflng c6 the dinh
nghii"nghich dAo" trong trudng hqp rdt dac bict.
Trong hinh hoc. ph6p nghich dio lh
^mQt ph6p
bien ddi hinh hoc.'mqt inh xa tU mat phing P den P vdi duong trbn d5 cho C (O, r) goi 1)r dudng trdn
nehich dio. Ph6p nghich dio cgc O v6i phuctng
tiitr (hay hq sO [ = 12 duq c xdc dinh nhu sau : fulh
cira didm M e P ld didm M' thu6c tta OM sao cho
OM x OM' =,2-. Didm O khong c6 anh. Anh cira M'ld M. C{c didm nam trong duong trdn c6 Anh nam ngoli duong trbn vd nguo. c l4i- Cic didrn thuoc i bat bien. Duong trdn bidn thhnh duong trbn n6u n5 khOng qua O. Ducrng thang bien thenh duong thang (chinh
clio hinh hoc mh "bii todn cd v6 ba dudng trbn"
cluoc giii nhe nhlng. Bdi to6n rtdy ld nQi dung
trao dti gifra Cong chria Elisabeth cira Thuy Didn vd nhl to6n hoc Ph6p Descartes vdo n6m 1643. Bei to6n li : "Vd dudng trbn thrl tu tidp xfc v6i ba dudng trbn C1, C2, Ct ti0p xric nhau
tung doi mot". Cong chria Elisabeth d6 giri cho