xcocnrBN
frong ki thi thu Dai hoc cria m6t truong
I THPT co mot bdi to6n vdin6idwrg: Trong niat nitit pltting t,6i he toa d6 On,, t,ho tam giric: ABC tr) phLro.ng tt.inh ttu.ing pkan
giitc {ront ,qrli, llAC lit (dt ", - -r, + 2 - 0 ,.
rlttclnc t:oo qrte rttnh B t:tt phuttng trinh (d');
.r..,-3r - 4-0 t't) ttiim A,{l*2;4) uinr t,,,,,
t'unlt ,41]. Bitlr ring a'1,1 = Jl0A, titn toa do
citr: dt'nh t'itt tcrni giitc.IBC.
MQt thi sinh cli hinh bdy ldi gi6i trOn nhu sau:
Ggi M' ld di6m dOi xrmg cua M quaphdn gi6c trong g6c BAC, H ld truc tdm tam gi6c.
Tim tluoc M'(2;0) (ta bi6t ring l| e AC).
Canh AC qua M' vit AC LBH ndn AC;
3x-y -6:0. Yi A= ADoAC, n}n A(4;6).
L
Qwdd.p bh)
HD 6 rang y ki6n cta bgn hgc sinh d6 ldJ-\kh6ng dring. Trong loi giai, bpn cl6 dd su J-\kh6ng dring. Trong loi giai, bpn cl6 dd su
dgng ph6p suy ludn y =logo x 3 yt= ,l
x.Eta
Luu y rang phgp suy luQn ndy chi 6p dung
dugc khi a ld hing s6.
Loi gi6i dring cua bdi to6n ndy c6 th6 6m
theo c6c huong sau:
o Htrong t. Ta co log2 3 > log3 4
<)logrgr.]^<)los.31 2eL loga 3 "-ot " log23 eL loga 3 "-ot " log23
*g \ \ B& bd\t 1- ;, J' <>logr3>Jr. (1)
5gi ilitii ?itu t tgt bdti te ern
TRONG ffiE T'E{I THUDAT HOC
Phuong trinh canh AB qua A vd M lit
x-3y+14=0. Ta c6 B:ABaBH n6n
B(-5; 3). Lai vi C e AC n6n C(x; 3x - 6), md
MC2 = 104 huy (x + 2)2 + (3x - t0;12 = 1g4. Giai PT ndy du<rc x:0; - ,5=+
YQy ta c6 hai tam gi6c thod mdn y6u cAu bdi
to6n ld: MBC vd, AABC, vorA(4;6);B(-5;3),
C(o;6)'u .'[?,+)
C6c ban thu binh lcri gi6i cua thi sinh d6 nh6!
f6t qud cira ban nhu th6 ndo?
NGITYEN VAN THOI
(GV THPT Nguydn Hiin, Duy X.ty€n, fudng Nam)
,, t'.
IeSl! FQC t'47'l., .w dc,*e,r6 ?
Iqp thi l0fill Hg( vu IUdl IRi
illuthemotic und louth ffioguzine
xuir nAx rtJ Dot
Dic san Sd'9
TOa soan : 1878, nho Gi6nq U6' He il9i 0I Bien lap: 04'351 21 607