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LuyÖn thi trªn m¹ng – Phiªn b¶n 1.0
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C©u 1.
7
1
) Víi a = , ta cã hÖ :
2
7
2
5
x + y + xy =
xy(x + y) =
2
§
Æt x + y = S, xy = P, th× ®−îc
7
5
S + P = , SP =
,
2
2
2
suy ra a) S = 1, P = 5/2 lo¹i v× kh«ng tháa m·n ®iÒu kiÖn S ≥ 4P ;
5
b) S = , P = 1 th× ®−îc nghiÖm
2
1
1
x = 2, y = ; x = , y = 2.
2
2
2
) Trong tr−êng hîp tæng qu¸t ta cã
S + P = a
SP = 3a −8
VËy S, P lµ nghiÖm cña ph−¬ng tr×nh
t2 − at + (3a − 8) = 0. (1)
iÒu kiÖn cña ph−¬ng tr×nh cã nghiÖm :
§
2 2
∆
= a − 4(3a − 8) = a − 12a + 32 ≥ 0 ⇒ a ≤ 4 hoÆc 8 ≤ a.
Víi ®iÒu kiÖn ®ã, ph−¬ng tr×nh (1) cã nghiÖm
2
2
a − a −12a + 32
a + a −12a + 32
t =
, t =
1
2
2
2
a) NÕu lÊy S = t , P = t , th× ph¶i cã ®iÒu kiÖn
1
2
2
2
S ≥ 4P ⇒ t ≥ 4t
1
2
2
2
2
hay
(a − a −12a + 32) ≥ 8(a + a −12a + 32)
2
2
⇒
a − 10a + 16 ≥ (a + 4) a −12a + 32 . (2)
2
b) NÕu lÊy S = t , P = t , th× t−¬ng tù nh− trªn, ph¶i cã t ≥ 4t hay
2
1
2
1
2
2
a −10a +16 ≥ −(a + 4) a −12a + 32 .
(3)
Thµnh thö ngoµi ®iÒu kiÖn a ≤ 4, 8 ≤ a, ®Ó hÖ cã nghiÖm, ta cßn ph¶i cã (2) hoÆc (3), tøc lµ
2
2
a −10a +16 ≥ − a + 4 a −12a + 32 .
(4)
2
V× a −10a +16 = (a − 2) (a − 8), nªn nÕu a ≤ 2, hoÆc 8 ≤ a th× (4) ®−îc nghiÖm. XÐt 2 < a ≤ 4,
2
khi ®ã a −10a +16 < 0, viÕt (4) d−íi d¹ng
a + 4 (a − 4)(a −8) ≥ − (a − 2)(a − 8),
c¶ hai vÕ ®Òu kh«ng ©m, cã thÓ b×nh ph−¬ng vµ ®−îc
2 2 2
(
(
a + 4) (a − 4)(a − 8) ≥ (a − 2) (a −8) hay do a − 8 < 0 :
2
2
2
a + 4) (a − 4) ≤ (a − 2) (a − 8) ⇒ 4a − 13a − 8 ≤ 0