: x2y2 z z (x+y+z)'
l) Tinh chiOt sudt tuv€t d6i ctia chcit tim thtu kinh.
D Dqt th&r kinh rrgdp m6r nira lrong nrrl.tr ,tcto
chct truc chfnh vudng got vr)'i nfit phcing phan =:+ ?r. ; *(*+'l+') y- +r- +y, \ - =3+(x+ r,+:) ' F ---i- fr, !' +z- +12 =3-(.r+r,+z)F , - \--'' -' *' *?,r(lr'+r'+yr) >3+(x+y+z (x+y+r)' f (y + x) + 1? (z + x) + / (x + y) $xy (*+y+r)' =3+(x+y+z) (x+y+z)(yz+zx+xl) Sdaez(rz-zors)'gH,HEEZS
cach nadc vd kh6ng khi.Tinh khodng cdch gifta hai fiAu diiim chinh.
Ldi gi,rti.
1) Khi dat th6u kinh trong kh6ng khi thi khoing c6ch gifia hai ti6u di6m chinh ld 2/
1)
= 7=@-I)fr t t) fu6i R ld b5n kinh m{t cong) Khi dil th6u kinh trong khdng khi thi khoing c6ch gita hai ti6u di€m chinhld2f2
| (n.)z
= r,=[;-tJ; Q)
rri (1) vd (2) ta c6 :
+ = #+=n =ff
nr-' i
2) Khi dit th6u kinh ngQp mQt ntra trong nu6c
' Xdt trudng hqp 1, m6i truong t6i ld kh6ng
thi,6p dgng cdng thfc luong ch6t cAu ta c6:
n_l I n nn_n
=!-*2 (4)
4-4 (j) -R = 4* 4
Vi th6u kinh li m6ng n6n: dr=-d* dt: @
Tir (3) vd (a) suy ra: ':,:n --l *n !' 61
.fr' R R
Thay (1) vd(2) vdo (5) ta c6:
fr, _ I fln ,' _2.fr.frn,
7-2f-4-rt-fr.rl
' Xdt trudng hqp 2, mdi trudng t6i ld nu6c,
tucrng t.u ta c6:
I _fr-ff, _n-1 : fr, _ | + /,_ 2.fr.[,
f;- R- R -2J;-2f,----zrz-fr.rf,
Suy ra khoing c6ch gita hai tiOu cti6m li:
tf : fi+f!:zf{',v:t) o
Jt + nJ,
YNhQn xdt. Chic mring c6c ban c6 loi gi6i dung d6
ra ki ndy: Nam Dinh: Nguydn Duc Dqt, llL!,
Nguydn Nam Khdnh, Phqm Ngoc Nam, Nguydn Vdn
Qudn, 12 Li, THPT chuyCn L6 H6ng Phong;
Khinh Hitaz Nguydn Kim Hodng, 10A12, THPT
Nguy6n Trdi, Ninh Da, Ninh Hod; Binh Phurfc:
Phetm Vdn Soz, AK11, THPT chuy6n a":"r tT"r.
DINH THI THAI QUYNH
Bni L2l458. M6t qud cdu l6n ddn di€n c6 bdn kinh R: 0,2 m daqc tich diQn d€n Z: 1000V.