ltr r'I
=fr =;i.Ket hqp voi IAE = I,AF
> LAI,F a LAEI (c.g.c).Do d6 AI,F = AEI (dpcm). d Do d6 AI,F = AEI (dpcm). d
Y NhQn xdt. Cic lcri gi6i giri vA Tda sopn kh6 phong
phri theo nhirng hu6ng: Sri dung tinh ch6t cua tth
gidc diiu hda; hdng di€m diiu hda; chilm diiu hda,
dinh ly Menelaus... MOt sti ban kd IKIIBC(KeAI),
goi X=DKa(O) (X*D) sau d6 tli chimg minh X, I, E thing hdng vd duong thtng DK di qua../ctng cho
lli gi6i gon. C5c bpn sau c6 loi gi6i t6t. Hn NQi: I/d
E*c Ydn, Phqm Ngpc Khdnh, llTodn l, Trin Bd Kh6i,11T2, THPT chuy6n DHSP HA NQi, Hodng LA NhQt Tilng,l1T2, THPT chuy6n KHTN, DHQG He
NQi; YOn Biiz Vil H6ng Qudn, 11To5n, THPT
chuy6n Nguy6n t6t ffrann; Phrri Thg: Nguydn Dttc
ThuQn,l0 Torin, THPT chuydn Hirng Vuong; Vinh
P}n{rc: Hd Hiiu Linh, 11A1, THPT chuy6n VTnh
Phric; Hnng YGn: Duong H6ng Son,1049, THPT
Ducrng Quing Hdm, V[n Giang; Nam D!nh: Dodn Thl Nhdi, llTo5n 1, THPT chuy6n L6 H6ng Phong;
Thanh IJl6a: Nguydn Dinh Luong, l0T, Vil Duy Manh, 11T, THPT chuy6n Lam Son; NghQ An: Cao Htbu Dat, Phan Dilrc Ti€n,llA1, THPT chuy€n Phan
BQi Chdu; Hi finh: Ng6 ViQt Hodng, Phan Anh
Tuiin, 11T1, THPT chuy6n Hd Tinh; Binh Phudc:
Bili Vdn Binh, Pham Nggc Huy, AKl1, Bili C6ng
Minh, l2A, THPT chuy6n Quang Trung, Dodn Thdnh Dqt, 12T1, THPT chuy6n Binh Long; Phri
I
YEnz LA Bdo Dqi, 11 To6n 2, THPT chuy6n Luong
Vdn Ch6nh; Cin Tho: Nguydn Trin HiLu Thinh, llAl, THPT chuyCn Ly TU Trong; TP. Hd Chi Minh: Nguydn Dinh Duy,l0 To6n, PTNK - DHQG
TP. HA Chi Minh.
HO QUANG VINH
BitiLll4s7. Oe do di€n trd R vd d0 tqr cdm L
cfia cuQn ddy, ta dilng cdu xoay chiiu n6i vao
,l -. ^
nguon diQn xoay chi€u c6 tdn s6 gdc a (Hinh
vq.C h m\t tu di€n c6 di€n dung dd bidt, h ld
diQn trd co gid tri dd biiit d hinh dadi, Rzvd r ld hai bi)n rd, r ldp riii Uap vdi C. nidn dih nzva
r dA cdu cdn biing (kh6ng cd ddng diQn qua T),
ta dgc &rqc Rz vd r. Goi cdc t6ng trd cila doan
AB, BD, AE, ED lin laqt ld. 26 22, Zs, Zq. a) Vd gidn dd Fre-nen. Suy ra *iii lie, hQ gifia