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2
Û x - 7x + 10 = 0 Û x = 2 hoÆc x = 5.
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2
2
Víi x = 2, ta cã log (2 - 12a ) = log
2
2 kh«ng ®óng víi mäi gi¸ trÞ cña a
+a
1
2
2
2
1
Ch¼ng h¹n khi a =
vÕ tr¸i b»ng 0 cßn vÕ ph¶i b»ng log2+(1/12) 2 ¹ 0.
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2
2
2
Víi x = 5, ta cã log (125a - 125a + 1) = log
2
1 ®óng víi mäi a.KÕt luËn : x = 5.
+a
2
2
2
C©u III. 1) V× a, b, c, d lËp thµnh mét cÊp sè céng, ta cã: a + d = b + c.
2
2
2
2
BiÕn ®æi : (x - a) (x - b) (x - c) (x - d) + m = [x - (a + d)x + ad] [x - (b + c)x + bc] + m =
2 2 2
(t + ad) (t + bc) + m = t + (ad + bc)t + m + adbc = f(t)
=
2
2
(
víi t = x - (a + d)x = x - (b + c)x).
XÐt tam thøc bËc hai f(t). Ta thÊy khi lÊy m sao cho
2 2
m ³ | ad - bc |, ta cã 4m ³ (ad - bc)
2
2
2
2
2
MÆt kh¸c, biÖt thøc ∆ = (ad + bc) - 4m - 4abcd = (ad - bc) - 4m .
2
2
Do ®ã ∆ £ 0, f(t) = t + (ad + bc)t + m + abcd ³ 0 víi mäi t.
ã lµ ®iÒu ph¶i chøng minh.
) Gäi A lµ vÕ tr¸i cña bÊt ®¼ng thøc ®· cho. Ta cã
§
2
2
a - b
2
2
b - c
2
2
2
c - a
A =
+
+
=
ab
bc
ca
1
2
2
2
2
2
2
c(a - b ) + a(b - c ) + b(c - a ) =
=
|
(a - b)(b - c)(c - a)|
|
abc
|
=
.
abc
V× a, b, c lµ c¸c ®é dµi c¹nh cña mét tam gi¸c, nªn
a - b | < c, | b - c | < a, | c - a | < b ; suy ra A < 1.
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