Sở giáo dục và đào tạo Hng Yên
Trờng THPT Kim Động
đề tài sáng kiến kinh nghiệm
rèn luyện t duy hàm
qua các bài tập giải phơng trình
bất phơng trình và
hệ phơng trình
Giáo viên: Đinh Văn Hữu
Đơn vị: Trờng THPT Kim Động
Kim động, tháng 5 - 2012
Phần 1: mở đầu
I. Lý do chọn đề tài:
!"#$#%&'() ) *+),-
./012034
'5 *+),6%
!!'1 78'
&."29:;0"<&+=>?@
29:*.1A.?!8.)+7*.
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&+.=D'5FE%=*G+,%B=
!2*8))7" 1.1,H
"Rèn luyện t duy hàm trong giải phơng trình, bpt và hệ phơng trìnhI'
II. Mục đích nghiên cứu:
-+3") /!J*KJ)LJB,!MN'
-ODP?8 !8"#=
-KQ%R") *1ST!'UD"#1!
T%*B'
III. Đối tợng nghiên cứu:
-(%B!J*KJ)LJV'
-J#B%BWG)!X%B'
IV. Phơng pháp nghiên cứu:
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B"' (H
-Y2J*KJ)LJ1E*6%8) 8,
=!'
-Y2J*KJ)LJDE*7) /).*-
*+) %BHZ<$>[GZ<$>\<GZ<$>]^Z<$>
^GZ<$>
>'_
D6%8) 8,=!'
Phần 2: Nội dung
I. Dạng 1: ứng dụng hàm số để giải phơng trình, bất phơng trình, và hệ phơng trình
Tính chất 1:
(HZ<$>[<$>$3`'
5./Z<$>G<$>,*aBV
G,0)21.D,,D%
'
Tính chất 2:
(Z<$>[$3`'
b 1,P)=D,/ 3=
Z<$>'
Tính chất 3:
(Z<$>[$3`
@
5.Z<$>),`D1
/,'
Tính chất 4:
(+HZ<$>\<Z<$>]>
>5.Z<$>,T`)QB$
:
`DZ<$
:
>[
7,=+JH[`
<$
:
^c
><[`
<-
^$
:
>>'
>5. Z<$>,!`)QB$
:
`DZ<$
:
>[7
,=+JH[`
<-
^$
:
><[`
<$
:
^c
>>'
Tính chất 5:
(Z<$>$3`
9'Z<$>
*
$
`
< >
x D
f x
@'Z<$>
*
$
`
$ < >
x D
f x
d'Z<$>
D,$
`
$ < >
x D
f x
e'Z<$>
D,$
`
< >
x D
f x
f'5.Z<$>,T`)QB*)
`'gDH
< > < >f u f v>
\)*Z<>[Z<)>
[)
h'5.Z<$>,!`)QB*)
`'gDH
< > < >f u f v>
])*Z<>[Z<)>
[)
1. ứng dụng hàm số để giải phơng trình
Phơng pháp :
Dạng 1HJi+.0) %B
< > < >f x g x=
<hoặc
< > < >f u g u=
>D
< >u u x=
'
K29HK.i) %B
< > < >f x g x=
<hoặc
< > < >f u g u=
>
K2@HjN
< >^ < >y f x y g x= =
`
k8
l
9
y
*$N%
l
9
y
*1.78,=
9
< >y f x=
`
k8
l
@
y
*$N%
l
@
y
*1.78,=
@
< >y g x=
`
kg.7
< >^ < >y f x y g x= =
,0*G/
V'
k
:
x
: :
< > < >f x g x=
<G
:
u
: :
< > < >f u g u=
>
K2dHg.7H
kJiD,1);1
:
x x=
<G
:
u u=
Q!>
d
kg.7,=i
Dạng 2HJi+.0) %B
< > < >f u f v=
D
< >u u x=
*
< >v v x=
K29HK.) %B
< > < >f u f v=
K2@HjN
< >y f x=
`
k8
ly
*$N%l
kg.7
< >y f x=
,`'
K2dHg.7H
kJiD,1);1
u v=
*!JH
u v=
kg.7,=i
2. ứng dụng hàm số để giải bất phơng trình
Phơng pháp :
Dạng 1HKJ+.) %B
< > < >f x g x>
<hoặc
< > < >f u g u>
>D
< >u u x=
'
K29HK.KJi) %B
< > < >f x g x>
<hoặc
< > < >f u g u>
>
K2@HjN
9 @
< >^ < >y f x y g x= =
`
k8
l
9
y
*$N%
l
9
y
*1.78,=
9
< >y f x=
`
k8
l
@
y
*$N%
l
@
y
*1.78,=
@
< >y g x=
`
k
:
x
: :
< > < >f x g x=
<G
:
u
: :
< > < >f u g u=
>
k5.Z<$>,T*<$>,!<GV>
:
< > < > *f x g x x x x D> >
<G
:
< > < > *f u g u u u x D> >
>
5.Z<$>,!*<$>,T<GV>
:
< > < > *f x g x x x x D> <
<G
:
< > < > *f u g u u u x D> <
>
K2dHg.7,=+i
Dạng 2HKJ+.0) %B
< > < >f u f v>
D
< >u u x=
*
< >v v x=
K29HK.+) %B
< > < >f u f v>
K2@HjN
< >y f x=
`
k8
ly
*$N%l'g.7
< >y f x=
,`'
k5.Z<$>,TH
< > < > *f u f v u v x D> >
e
5.Z<$>,!H
< > < > *f u f v u v x D> ⇔ < ∈
K2dHg.7,=+i
Bµi 1:O!H
'
9 h @ hx x x+ + + + − =
+'
@ f 9
9 9
@ f 9
x x
e e
x x
− −
− = −
− −
'm
@
<$
@
-$cf>[d<$
@
-$cf>
%'
@
9 @
@ @ < 9>
x x x
x
− −
− + = −
2.*71!0+V
WGD!0n1D1T'o6%
!'
Gi¶i:
'
9 h @ hx x x+ + + + − =
jbH
[
)
@^cD = ∞
jNH
< > 9 h @f x x x x= + + + + −
cjbH
[
)
@^cD = ∞
cbBH
9 9 9
l< > :* @
@ 9 @ h @ @
f x x
x x x
= + + > ∀ >
+ + −
`D
< >f x
Q+.`*)7.D,,D%
'
pG1DHZ<d>[h'Y7D,%$[d'
+'
@ f 9
9 9
@ f 9
x x
e e
x x
− −
− = −
− −
b 1,H
@ f : f q @
9 : 9
x x
x x
− ≠ ≠
⇔
− ≠ ≠
Y.B%2%BH
@ f 9
9 9
@ f 9
x x
e e
x x
− −
− = −
− −
<9>
jN
9
< >
t
f t e
t
= −
)2\:
f
cbBH
@
9
l< > :* :
t
f t e t
t
= + > ∀ >
⇒
L
< >f t
Q+.1!
<:^ >+∞
'
gDH<9>
⇔
< @ f > < 9>f x f x− = −
⇔
@ f 9x x− = −
⇔
@ f 9 e
@ f 9 @
x x x
x x x
− = − =
⇔
− = − + =
Y7D,$[@)$[e'
'm
@
<$
@
-$cf>[d<$
@
-$cf><9>
Y2D#!+.
0$N'
jbH`[
¡
`<9>
⇔
@
@
@
< f>
d
m
f
x x
x x
− +
=
− +
<%
@
fx x− +
\\:>
bG[
@
fx x− +
)2\*?H
@
d
m
t
t
=
<@>
jNH
@
< >
t
f t
t
=
)2\
D
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9
l< >
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t
f t
t
−
=
]:
∀
\
rD*).=<@>3+.
∀
\^).!V
`D<@>.D,D,%'
pG1
d
<m>
m
f =
⇒
J<@>D,%[m
Y2[mD
@
f mx x− + =
⇔
$[
9 9d
@
+
^$[
9 9d
@
−
Y7iD@,$[
9 9d
@
+
^$[
9 9d
@
−
%'
@
9 @
@ @ < 9>
x x x
x
− −
− + = −
<9>
&#>)2nP+.$,P$N'
jbH`[
¡
`^<9>
⇔
@
9 @
@ @ @ 9
x x x
x x
− −
− + = − +
⇔
@
9 @
@ 9 @
x x x
x x x
− −
+ − = + −
h
jN
< > @
t
f t t= +
)2
∈
¡
s< > @ '@ 9 :tf t = + >
∀
t
∈
¡
⇒
Z<>Q+.
¡
pG1<9>
⇔
Z<$-9>[Z<$
@
-$>
⇔
$-9[$
@
-$
⇔
$
@
-@$c9[:
⇔
$[9
Y7iD,%$[9
Bµi 2HO!+H
'
h @ e dx x x+ + − − − >
+'
@ d @
e @ 9 < 9> h 9f 9ex x x x x x− − + > − + −
'
@ d
9 u 9x x+ + + >
%'
@< 9> 9 @
d d e d
x x
x x
− +
− ≤ − +
Gi¶i:
'
h @ e dx x x+ + − − − >
jbH`[
[ ]
@^e
jNHZ<$>[
h @ ex x x+ + − − −
)2$
∈
`
n7Z<$>Q+.`<)Zs<$>\:
∀
x
∈
<@^e>>
vBDHZ<d>[d^%D*+D,$
<d^ >x ∈ +∞
'Y77,H[
[ ]
@^e
∩
<d
^c
∞
>[
(
]
d^e
+'
@ d @
e @ 9 < 9> h 9f 9ex x x x x x− − + > − + −
<9>
jbH`[
¡
*KJ<9>
⇔
@ d
@ 9 <@ 9> d < @> d hx x x x
− − + > − + −
⇔
d
d
@ 9 d @ 9 < @> d< @>x x x x− + − > − + −
<@>
jNH
d
< > df x x x= +
Q+.
¡
'
gDH<@>
⇔
< @ 9> < @> @ 9 @f x f x x x− > − ⇔ − > −
⇔
@ 9 @ 9
@ 9 @ 9
x x x
x
x x x
− > − > −
⇔ ⇔ ∀
− < − + <
∈
¡
Y7+,C)2"$
∈
¡
'
'
@ d
9 u 9x x+ + + >
<9>
w
b 1,H$\-9*
9 @
< > 9f x x= +
)
@ d
< > uf x x= +
Q+.1!
< 9^ >− +∞
*
@ d
< > 9 uf x x x= + + +
Q+.1!
< 9^ >− +∞
'
pG1
<:> 9f =
)7<9>
< > <:> :f x f x⇔ > ⇔ >
'
Y7,=+$\:'
%'
@< 9> 9 @
d d e d
x x
x x
− +
− ≤ − +
<9>
b 1,H
9 : 9x x− ≥ ⇔ ≥
'Y7jbH`[
[
)
9^+∞
<9>
⇔
@< 9> 9 @
d @< 9> d @ 9
x x
x x x
− +
+ − ≤ + − +
⇔
@< 9> 9 < 9> 9 @
d @< 9> d < 9>
x x
x x
− + − +
+ − ≤ + −
<@>
jN
9 @
< > d
t
f t t
+
= +
*Q+.`'
Y7`^<@>
⇔
< @< 9>> < 9> @< 9> 9f x f x x x− ≤ − ⇔ − ≤ −
⇔
@
@< 9> < 9> *< 9>x x do x− ≤ − ≥
⇔
@
e d :x x− + ≥ ⇔
$[9G$
≥
d'
Y7,=+$[9)
∀
$
≥
d'
Bµi 3: O!,H
@
@
d @ d
d @ d
x x y
y y x
+ + = +
+ + = +
Gi¶i:
b 1,
:* :x y≥ ≥
'L,i?H
@
@ @
@
d @ d
d d d d d d <9>
d d @
x x y
x x y y
x y y
+ + = +
⇒ + + + = + + +
+ = + +
jN
@
< > d d df t t t= + + +
cjbH
[
)
:^D = +∞
cbB
@
d
l< > :* :
@
d
t
f t t
t
t
= + > ∀ >
+
Q+.`'
Y7`*<9>0).%2%B
< > < >f x f y x y= ⇔ =
'
gD,i?
@
@
d @ d
d d <@>
x x y
x x
x y
x y
+ + = +
+ = −
⇔
=
=
m
O!<@>H0$[9/,=<@>*G1%x7<@>D).
Q+.*).!3+.'
Y7$[9,%=J<@>*Y7,iD,%$[[9'
NhËn xÐt:b)2,*,+ y+.$,-
!0+V) ,Fy!Q
zrW0!.'
NhËn xÐtHb)2!,*,+D9yD%{
!r+=,Q1.07,01.7)
,,+'
II. D¹ng 2: Sö dông hµm sè ®Ó biÖn luËn ph¬ng tr×nh
Bµi 4: K,7,=H
>
@
e d
@
x
x x m− + = +
+>
@ @ d @
9 < @ @> d e @mx m x mx x x x+ + + = − + −
>
@
h e d @
@ @ <e > d h
m x x m
m x m
+ +
− = − + −
%>
@ @
@ 9
@
d @ < > d @x x x m x x− + + − + − +
-$c[:
Gi¶i:
>
@
e d
@
x
x x m− + = +
<9>
NhËn xÐt:K7D!+VW'*.
!+VD*!1 1,=yEB'o
!++V6%
Gi¶i:jbH`[
(
] [
)
^9 d^−∞ ∪ + ∞
`^<9>
⇔
@
e d
@
x
x x m− + − =
jNZ<$>[
@
e d
@
x
x x− + −
)2$
∈
`
DHZs<$>[
@
@ 9
@
e d
x
x x
−
−
− +
`DHZs<$>\:
⇔
@
@ 9
@
e d
x
x x
−
−
− +
\:
⇔
$\d^
Zs<$>]:
⇔
@
@ 9
@
e d
x
x x
−
−
− +
]:
⇔
$]9
u
rD*D+!+.H
$ -
∞
9 d c
∞
Zs<$> - c
Z<$>
_,=<9>=Q3[Z<$>)W|[
'
`&)+!+.D1.!+,7H
-5.]
d
@
−
*W|[1AQ3[Z<$>*%D
<9>),'
-5.
d
@
−
≤
]
9
@
−
*W|[AQ3[Z<$>B9*%D-
<9>D9,'
-5.
≥
9
@
−
*W|[AQ3[Z<$>B@*%D
<9>D@,'
b)
@ @ d @
9 < @ @> d e @mx m x mx x x x+ + + = − + −
Y.B%2%B
@ d
9 < 9> 9 < 9> < 9>mx mx x x
+ + + = − + −
d
d
9 9 < 9> < 9>mx mx x x⇔ + + + = − + −
<@>
jN
d
< >f t t t= +
Q+.
∈
¡
Y7<@>
< 9> < 9> 9 9f mx f x mx x⇔ + = − ⇔ + = −
9 9
< >
9 9 < 9> @ <d>
9 9
<e> < >
9 9 < 9> :
x x
I
mx x m x
x x
II
mx x m x
≥ ≥
+ = − − = −
⇔ ⇔
≥ ≥
+ = − + + =
cO!)+,7<}>
-Y2[9<d>),<}>),
-Y2
≠
9<d>D,
@
9
x
m
= −
−
*
9:
2
1
−
+
+
2
3
−
D,=<}>1
@
9 9 9 9
9
x m
m
≥ ⇔ − ≥ ⇔ − ≤ <
−
cO!)+,7<}}>
-Y2[-9<e>,C)2"$*<}}>7$
≥
9,
-Y2
≠
-9<e>D,$[:*1,=<}}>
g.7H
-Y2]-9G
≥
9H),
-Y2[-9HD,
∀
$
≥
9
-Y2-9]]9HD,
@
9
x
m
= −
−
>
@
h e d @
@ @ <e > d h
m x x m
m x m
+ +
− = − + −
<9>
Y.B%2%B
@
h @ e d
@ h @ e d
m x x m
m x x m
+ +
+ + = + +
<@>
jN
< > @
t
f t t= +
Q+.
¡
*)7<@>
@ @ @
< h> <e d > h e d < e> d h <d>f m x f x m m x x m m x m⇔ + = + ⇔ + = + ⇔ − = −
-5.
@
e : @m m− = ⇔ = ±
cY2[@*<d>
⇔
:'$[:*,C)2
x∀ ∈¡
cY2[-@*<d>
⇔
:'$[-u*),
-5.
@
e : @m m− ≠ ⇔ ≠ ±
J<d>D,%
d
@
x
m
=
+
g.7H
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@m ≠ ±
HD,%
d
@
x
m
=
+
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x R∀ ∈
-Y2[-@H),'
%>
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@ 9
@
d @ < > d @x x x m x x− + + − + − +
-$c[:<9>
Y.B%2%BH
@ @
@ @
d @ d @ < >x x x x x m x m− + + − + = − + −
<@>
99
b 1,H$
@
-d$c@\:
⇔
$]9G$\@'jbH`[
< ^9> <@^ >−∞ ∪ +∞
jN
@
< > f t t t= +
Q+.1!
<:^ >+∞
'Y7`*
<@>?H
@ @
< d @> < > d @f x x f x m x x x m− + = − ⇔ − + = −
@
:
< >
<@ d> @ <d>
x m
I
m x m
− >
⇔
− = −
K,7H
-Y2@-d[:
⇔
d
@
m =
*1D<d>),<}>),
-Y2
d
@ d :
@
m m− ≠ ⇔ ≠
*1D<d>D,%
@
@
@ d
m
x
m
−
=
−
*
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@ @
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:
@ d @ d
m m m
m
m m
− − +
> ⇔ <
− −
9
d
@
@
m
m
<
⇔
< <
g.7H
-Y2
( )
d
^9 ^@
@
m
∈ −∞ ∪
÷
D,
@
@
@ d
m
x
m
−
=
−
-Y2
[
)
d
9^ @^
@
m
∈ ∪ +∞
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III. D¹ng 3: Sö dông hµm sè t×m ®iÒu kiÖn cña tham sè ®Ó ph¬ng tr×nh, bpt tho¶ m·n ®iÒu
kiÖn cho tríc.
Bµi 5:D,H
@
@ @< e> f 9: d :x m x m x− + + + + − =
<9><->
Gi¶i: <9>
⇔
@
@ @< e> f 9:x m x m− + + +
= $-d
⇔
@ @
d :
@ @< e> f 9: < d>
x
x m x m x
− ≥
− + + + = −
9@
⇔
@
d
@< 9> f 9 :
x
x m x m
≥
− + + + =
⇔
@
d
@ 9
<@>
@ f
x
x x
m
x
≥
− +
=
−
J<9>D,
⇔
<@>D,!i$
≥
d
?+D6%E+7!'o6%
!+'
jN<@>HbGZ<$>[
@
@ 9
@ f
x x
x
− +
−
)2$
≥
d
DHZs<$>[
@
@
@ 9: m
<@ f>
x x
x
− +
−
Zs<$>[:
⇔
9
e
x
x
=
=
D+!+.H
$ -
∞
d e c
∞
Zs<$> -: c
Z<$>
`&)+!+.H
J<@>D,$
≥
d
⇔
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V. Mét sè bµi tËp tù gi¶i:
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Phần 3: Kết luận
I - kết luận:
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V. hớng nghiên cứu mở rộng đề tài
b#0"7="o.)7% 1%BbB
)O!LJEE%8=)/
+E+|E
II - kiến nghị:
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Kim Động 5 - 2012
GV: Đinh Văn Hữu
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