Dïng ®¹o hµm ®Ó chøng minh ®¼ng thøc tæ hîp
Bµi tËp:
1. Chøng minh r»ng:
0
2
4
2010
C2011
32.C2011
34.C2011
... 32010.C2011
2 2010 (2 2011 1)
Tõ ®ã tæng qu¸t lªn b»ng c¸ch thay 2011 bëi mét sè tù nhiªn n bÊt k×.
2. Cho n lµ sè tù nhiªn. Chøng minh ®¼ng thøc sau:
1
3
5
2009
1
4
6
2010
C2010
3C2010
5C2010
.. 2009C2010
2C2010
4C2010
6C2010
.. 2010C2010
3. Cho n lµ sè tù nhiªn lín h¬n 1. Rót gän biÓu thøc sau:
n
k 1
2n k .k .Cnk 2n 1 Cn1 2.2n 2 Cn2 3.2n 3 Cn3 ... nCnn
4. T×m sè tù nhiªn n biÕt r»ng:
C21n 1 2.2.C22n 1 3.22.C23n 1 ... (1) k 1.2 k 1.C2kn 1 ... (2 n 1).2 2 n.C22nn11 2011
5. Chøng minh r»ng:
99
100
101
0 1
1 1
2 1
100C100
101C100 102C100
2
2
2
2
100
b»ng c¸ch xÐt khai triÓn ( x x) .
199
100 1
... 200C100
2
0
6. Chøng minh r»ng ®¼ng thøc sau ®óng víi mäi sè tù nhiªn n lín h¬n 2:
2.1.Cn2 3.2.Cn3 ... n.(n 1).Cnn n( n 1).2n 2
7. B»ng c¸ch xÐt khai triÓn ( x 1) n , chøng minh r»ng ®¼ng thøc sau ®óng víi mäi n:
n 2Cn0 (n 1)2 Cn1 (n 2)2 .Cn2 ... (1) n 1 Cnn 1 0 .
Sö dông ®¹o hµm ®Ó chøng minh ®¼ng thøc
Lêi gi¶i
8. Chøng minh c¸c ®¼ng thøc lîng gi¸c trong tam gi¸c:
a. Ta cÇn chøng minh: sin A sin B sin C 4cos
A
B
C
cos cos 0 .
2
2
2
Do A, B, C lµ c¸c gãc cña mét tam gi¸c nªn: A B C C ( A B ) .
Cè ®Þnh B, ta xÐt hµm sè biÕn A nh sau:
( A B )
A
B
cos cos
2
2
2
A
B
A B
sin A sin B sin( A B ) 4cos cos sin
2
2
2
f ( A, B ) sin A sin B sin ( A B ) 4cos
Ta sÏ chøng minh ®¹o hµm cña hµm sè nµy b»ng 0 víi mäi A. ThËt vËy:
B 1
A
A B 1
A
A B
sin sin
cos cos
2 2
2
2
2
2
2
B
2A B
Suy
cos A cos( A B ) 2cos cos
2
2
A ( A B)
A ( A B)
cos A cos( A B ) 2cos
cos
0
2
2
ra víi B cè ®Þnh th× f ( A, B) lµ hµm h»ng víi mäi A. Cho A 0 , ta cã:
f ( A, B) cos A cos( A B) 4cos
0
B
B
B
B
f (0, B ) sin 0 sin B sin B 4cos cos sin 2sin B 4sin cos 0 .
2
2
2
2
2
VËy f ( A, B ) 0, A, B (0, ) . Ta cã ®pcm.
b. XÐt hµm sè: f ( A, B ) cos A cos B cos( A B ) 4sin
A
B
A B
sin cos
1 .
2
2
2
T¬ng tù c©u a., ta chøng minh f ( A, B) 0 vµ f (0, B) 0 .
c. XÐt hµm sè: f ( A, B ) tan
A
B
B
A B
A B
A
tan tan cot
cot
tan 1 .
2
2
2
2
2
2
T¬ng tù c©u a., ta chøng minh f ( A, B) 0 vµ f (0, B) 0 .
9. Chøng minh ®¼ng thøc:
(a) arcsin x arccos x
x [1, 1];
,
2
Víi x 1, arcsin1 arccos1
0 .
2
2
Víi x 1, arcsin(1) arccos(1)
.
2
2
Suy ra ®¼ng thøc trªn ®óng trong trêng hîp x 1 .
XÐt hµm sè: f ( x) arcsin x arccos x
1
(arccos)
f ( x)
1 x2
1
1 x
2
, (arcsin)
1
1 x
2
1
1 x2
, x ( 1,1) . Ta cã:
2
. Suy ra:
0 0 . Suy ra, lµ hµm h»ng víi mäi x (1,1) .
2
2
2
Cho x 2 , ta cã: f ( ) arcsin
arccos
0.
2
2
2
2
2
Do ®ã: f ( x) 0, x (1,1) .
Tõ ®ã suy ra: arcsin x arccos x
(b) arctan x arccot x
0, x [1,1] . Ta cã ®pcm.
2
, x �.
2
Còng t¬ng tù c©u (a), ta xÐt hµm sè: f ( x ) arctan x arccot x
Chó ý r»ng (arctan)
Suy ra: f ( x)
, x �.
2
1
1
.
, (arc cot)
2
1 x
1 x2
1
1
0 0 , tøc lµ hµm h»ng víi mäi x �.
2
1 x 1 x2
H¬n n÷a f (1) arctan 1 arccot1
0.
2 4 4 2
Do ®ã: f ( x) 0, x �. Ta cã ®pcm.
10. XÐt khai triÓn:
( x 1) n Cnn Cnn 1 x Cnn 2 x 2 ... (1)n 1 Cn1 x n 1 (1)n Cn0 x n
§¹o hµm hai vÕ theo biÕn x, ta cã:
n ( x 1) n 1 Cnn 1 2 Cnn 2 x ... ( 1) n1 ( n 1) Cn1 x n 2 ( 1) n n Cn0 x n 1
Nh©n hai vÕ cña biÕu thøc trªn cho x, ta ®îc:
n x ( x 1) n1
Cnn1 x 2 Cnn 2 x 2 ... ( 1) n 1 (n 1) Cn1 x n1 (1)n n Cn0 x n
TiÕp tôc lÊy ®¹o hµm hai vÕ theo biÕn x, ta cã:
n (n 1).x ( x 1) n2 n.( x 1) n1
Cnn1 22 Cnn 2 x ... (1) n 1 (n 1)2 Cn1 x n 2 (1) n n 2 Cn0 x n1
Cho x 1 , ta ®îc:
0 Cnn1 22 Cnn 2 ... (1)n 1 (n 1) 2 Cn1 (1)n n 2 Cn0 hay
(1) n n 2 Cn0 (1) n 1 (n 1)2 Cn1 ... 22 Cnn 2 Cnn 1 0
Ta cã ®pcm.
11. XÐt khai triÓn:
0
1
2
2
2009
2010
(1 x) 2010 C2010
C2010
x C2010
x 2 C2010
x 2 ... C2010
x 2009 C2010
x 2010
§¹o hµm hai vÕ theo biÕn x, ta ®îc:
1
2
2009
2010
2010 (1 x) 2009 C2010
2 C2010
x ... 2009 C2010
x 2008 2010 C 2010
x 2009
Cho x 1 , ta cã:
1
2
2009
2010
0 C2010
2 C2010
... 2009 C2010
2010 C2010
1
3
2009
2
4
2010
C2010
3C2010
L 2009C2010
2C2010
4C2010
L 2010C2010
.
Ta cã ®pcm.
12. XÐt khai triÓn:
100
99
98
1
0
( x 2 x)100 C100
x 200 C100
x198 x C100
x196 x 2 ... C100
x 2 x99 C100
x100
100
99
98
1
0
C100
x 200 C100
x199 C100
x198 ... C100
x101 C100
x100
§¹o hµm hai vÕ theo biÕn x, ta ®îc:
100 (2 x 1) ( x 2 x)99
100
99
98
1
0
200 C100
x199 199 C100
x198 198 C100
x197 ... 101 C100
x100 100 C100
x 99
1
Cho x , ta cã:
2
199
0 200 C
100
100
1
2
99
198
199 C
99
100
100
1
1 1
100C 101C100
2
2
0
100
100
99
1
1
1
... 101 C100
2
2
100 C
0
100
101
1
102C
2
2
100
1
2
199
L 200C
100
100
1
2
0.
Ta cã ®pcm.
13. T×m sè tù nhiªn n biÕt r»ng:
C21n 1 2·2·C22n1 3·22·C23n 1 L (1) k 1·2 k 1·C2kn1 L (2n 1)·22 n ·C22nn11 2011.
Tríc hÕt, ta sÏ rót gän:
Sn C21n 1 2·2·C22n1 3·22·C23n 1 L (1) k 1·2k 1·C2kn1 L (2n 1)·22 n ·C22nn11
XÐt khai triÓn:
(1 x) 2 n1 C20n1 C21n1 x C22n 1 x 2 ... C22nn1 x 2 n C22nn11 x 2 n 1 .
LÊy ®¹o hµm hai vÕ theo biÕn x, ta ®îc:
(2n 1) (1 x) 2 n C21n 1 2 C22n 1 x ... 2n C22nn1 x 2 n1 (2n 1) C22nn11 x 2 n
Cho x 2 , ta ®îc:
2n 1 C21n 1 2 2 C22n 1 ... 2n 22 n 1 C22nn1 (2n 1) 2 2 n C22nn11 .
Do ®ã, víi mäi sè tù nhiªn n th×: S n 2n 1 .
Suy ra: Sn 2011 2n 1 2011 n 1005 .
VËy gi¸ trÞ n cÇn t×m lµ 1005.
14. Ta cÇn tÝnh:
Sn 2n1 Cn1 2·2n 2 Cn2 3·2n 3 Cn3 L nCnn .
2
n 1
S
1
1
1
Ta thÊy: nn1 Cn1 2 Cn2 3 Cn3 L n Cnn
2
2
2
2
XÐt khai triÓn:
( x 1) n Cn0 Cn1 x Cn2 x 2 ... Cnn 1 x n 1 Cnn x n
§¹o hµm hai vÕ theo biÕn x, ta cã:
n ( x 1) n 1 Cn1 2 Cn2 x ... ( n 1) Cnn 1 x n1 n Cnn x n1 .
n 1
1
3
Cho x , ta cã: n
2
2
n2
1
1
C 2 C ... (n 1) Cnn 1
2
2
1
n
2
n
n 1
Suy ra:
Sn
3
n
n 1
2
2
S n n 3n 1 .
n1
1
n C
2
n
n
.
VËy tæng cÇn tÝnh lµ: S n n 3n1 .
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