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BI TON TNG QUT
kho st hm s sau
3 2y f(x) ax bx cx d,(a 0)
Ta ln lt c
Tp xc nh: D
Gii hn 3 2x x
,a 0lim f(x) lim (ax bx cx d)
,a 0
o hm 2y ' 3ax 2bx c
2y ' 0 g(x) 3ax 2bx c 0
Ta c 2' g(x) b 3ac
Nu ' g(x) 0 hm s khng c cc tr
Nu ' g(x) 0 hm s c 2 cc tr
Lu : hm s bc 3 khng c trng hp c 1 cc tr
o hm cp 2 y'' 6ax 2b
by '' 0 6ax 2b 0 x
3a
Lp bng bin thin
Kt lun
Ty theo du ca h s a v du ca 2b 3ac m ta c kt lun khc nhau v cc tr v cc khong ng bin nghch bin ca hm s. th
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hm s nhn 2 3
2
b 27a d 9abc 2bU ,
3a 27a lm im un v cng l
tm i xng ca th
th:
Ty thuc vo bng bin thin m ta c 6 dng th khc nhau ca
hm s bc 3.
Mt s tnh cht ca hm bc 3 cn nh l
Tnh cht 1: hm s ng bin (nghch bin) trn khi v ch khi
2
a ( )0
b 3ac 0
Chng minh: da vo vic bin lun du ca y ' v nh l o du tam
thc bc 2. Ta c ngay iu phi chng minh.
Tnh cht 2: hm s c cc i v cc tiu khi v ch khi 2b 3ac 0
v nu gi 0x l cc tr ca hm s th gi tr cc tr ca hm s l
2
0 0
6ac 2b 9ad bcy(x ) x
9a 9a
Chng minh: hm s c 2 cc tr th phng trnh y ' 0 c 2
nghim phn bit. Lp bit thc delta ta c ngay iu kin l y ' 0.
Gi 0x l cc tr ca hm s ta c
0y '(x ) 0 do ta c
23 2 2
0 0 0 0 0 0 0
2
0 0
3ax b 6ac 2by(x ) ax bx cx d (3ax 2bx c) x
9a 9a9ad bc 3ax b 6ac 2b 9ad bc
y '(x ) x9a 9a 9a 9a
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20
6ac 2b 9ad bcx
9a 9a
Do ta c 2
0 0
6ac 2b 9ad bcy(x ) x
9a 9a
Tnh cht 3: th hm s bc 3 nhn im un U lm tm i xng
Tht vy, tnh tin h trc ta v im un U ta c ngay cng thc di trc l
2 3
2
bx X
3a27a d 9abc 2b
y Y27a
Thay vo hm s ta s c
3 22 3
2
23
27a d 9abc 2b b b bY a X b X c X d
3a 3a 3a27a3ac b
Y aX X3a
Nhn xt hm s 2
3 3ac bY aX X3a
l mt hm s l nn th
hm s s nhn im U lm tm i xng.
Tnh cht 4: tip tuyn vi th hm s ti im un c h s gc nh
nht khi a 0 v c h s gc ln nht khi a 0.
Ta c h s gc ca tip tuyn bt k vi th hm s ti im 0
x x
l
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2 22 2
0 0 0 0 0
22 2
20
2
min 0
2
max 0
b b 3ac bk y '(x ) 3ax 2bx c 3a x 2 x
3a 9a 3a
3ac bk , a 0b 3ac b 3a3a x3ac b3a 3a
k , a 03a
3ac b ba 0,k x
3a 3a3ac b b
a 0,k x3a 3a
Do ta c iu cn phi chng minh
Tnh cht 5: th hm s ct trc honh ti 3 im cch u nhau hay
honh cc giao im lp thnh cp s cng khi v ch khi 2 327a d 9abc 2b 0 .
Tht vy ta c phng trnh honh giao im gia th hm s v
Ox l
3 2ax bx cx d 0
Gi s th hm s ct Ox ti 3 im phn bit th phng trnh trn
s c 3 nghim tha mn 1 2 3
1 3 2
x x x
x x 2x. Mc khc theo nh l Viete
ta c 1 2 3 2 2
b b bx x x 3x x
a a 3a
Mc khc 2x l nghim ca phng trnh 3 2ax bx cx d 0 do
ta c
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3 2
3 2
2 2 2
2 3
U
b b bax bx cx d 0 a b c d 0
3a 3a 3a
27a d 9abc 2b 0 y 0
Do ta c iu phi chng minh.
Lu : iu kin trn ch l iu kin cn khng phi l iu kin nn
nu bi ton yu cu tm iu kin tham s th hm s tha mn
tnh cht trn th sau khi tm c tham s cn phi kim tra li
Tnh cht 6: nu th hm s ct Ox ti 3 im c honh lp thnh
cp s nhn th 3 3ac b d 0
Tht vy ta c phng trnh honh giao im ca th hm s
cho vi Ox l
3 2ax bx cx d 0
V th ct Ox ti 3 im c honh lp thnh cp s nhn nn ta c
phng trnh 3 2ax bx cx d 0 s c 3 nghim tha mn 2
1 3 2x x x . Mc khc theo nh l Viete ta c
2
1 2 2 3 3 1 1 2 2 3 2
2 1 2 3 2 2
c cx x x x x x x x x x x
a ac b c c
x (x x x ) x xa a a b
V 2x l nghim ca phng trnh 3 2ax bx cx d 0 nn ta c
3 2
3 2
2 2 2
c c cax bx cx d 0 a b b d 0
b b b
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3 33 3
3
ac b d0 ac b d 0
b
Do ta c iu phi chng minh. Lu cng nh tnh cht 5, tnh cht
6 cng ch cho ta iu kin cn.
Tnh cht 7: gi th hm s ct Ox ti 3 im phn bit, gi 1 2S ,S l
din tch hnh phng gii hn gia th v Ox (phn nm pha trn v
pha di trc Ox ) Nu 1 2S S th im un U thuc trc honh
Ta c nu im un thuc trc honh th 1 2S S , bng phng php
dch chuyn th ta s c iu cn chng minh
Tnh cht 8: gi s hm s 3 2y ax bx cx d c th l (C).
Trn (C) ta ly 3 im M,N,P sao cho M,N,P thng hng. Tip tuyn
vi (C) ti M,N,P ln lt l M N P(t ),(t ),(t ). Cc tip tuyn
M N P(t ),(t ),(t ) ct th (C) ti 3 im khc l I,J,K th ta s c 3 im
I,J,K cng thng hng
Ta c ta cc im M,N,P ln lt l 3 2(m,am bm cm d), 3 2(n,an bn cn d) v 3 2(p,ap bp cp d) khi ta c 3 im
M,N,P s ng vi 3 s phc sau 3 2Mz m i(am bm cm d) ,
3 2
Nz n i(an bn cn d) v 3 2
Pz p i(ap bp cp d).
V cc im M,N,P thng hng nn ta c
M N M N
P N P N3 3 3 3 3 3
2 3 3 2 2 2
z z z zIm 0
z z z z
(am an bm bn cm cn )(p n)
(p n) ( an ap bn bp cn cp)
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3 3 2 2
2 3 3 2 2 2
3 3 3 3 3 3
3 3 2 2
(m n)( an ap bn bp cn cp)
(p n) ( an ap bn bp cn cp)(am an bm bn cm cn )(p n)
(m n)( an ap bn bp cn cp) 0
(n p)(m p)(m n)(am an ap b) 0
V cc im M,N,P phn bit nn m n p do ta c
(m n)(n p)(m p) 0 am an ap b 0
Phng trnh tip tuyn vi (C) ti M l 2 3 2
M(t ) : y (3am 2bm c)(x m) am bm cm d
Phng trnh honh giao im ca M(t ) v (C) l
3 2 2 3 2
2
I
2 3 2 2
I
2 3 2 2
ax bx cx d (3am 2bm c)(x m) am bm cm d
(x m) (2am ax b) 0
x m2am b
x2am bax
a8a m 8abm 2acm 2b m ad bc
ya
2am b 8a m 8abm 2acm 2b m ad bcI ,
a a
Tng t ta cng s c
2 3 2 22an b 8a n 8abn 2an 2b n ad bcJ ,
a a
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2 3 2 22ap b 8a p 8abp 2ap 2b p ad bcK ,
a a
Do ta s c 3 s phc tng ng vi 3 im I,J,K l
2 3 2 2
I
2 3 2 2
J
2 3 2 2
K
2am b 8a m 8abm 2am 2b m ad bcz i
a a2an b 8a n 8abn 2an 2b n ad bc
z ia a
2ap b 8a p 8abp 2ap 2b p ad bcz i
a a
Ta c
2I J
K J
z z 4(m p)(m n)(am an ap b)aIm 0z z (m,n,p)
Do ta c I,J,K thng hng.
Tnh cht 9: bin lun s nghim ca phng trnh bc 3
Xt phng trnh bc 3 sau 3 2ax bx cx d 0
Ta c 2 cch lm thun i s-gii tch nh sau
Cch 1: cch lm thun i s
Gi s ta bit c 0
x x l mt nghim ca phng trnh trn khi
ta c
2 2
0 0 0 0
02 2
0 0 0
(x x )[ax (ax b)x ax bx c] 0
x x
g(x) ax (ax b)x ax bx c 0
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Do ta c
3 2ax bx cx d 0 c 1 nghim duy nht th iu kin l
phng trnh g(x) 0 v nghim hoc c nghim kp l 0x . Do ta
c iu kin s l
2 2 2
0 0
2 2 2
0 02
0 0 0
3a x 2(bx 2c)a b 0g(x) 0
g(x) 0 3a x 2(bx 2c)a b 0
g(x ) 0 3ax 2bx c 0
Vy iu kin l
2 2 2
0 0
2 2 2
0 02
0 0
3a x 2(bx 2c)a b 0
3a x 2(bx 2c)a b 0
3ax 2bx c 0
phng trnh 3 2ax bx cx d 0 c 2 nghim th phng trnh
g(x) 0 phi c 2 nghim phn bit v mt nghim trng vi 0x . Do
ta c ta c iu kin l
2 2 2
0 02
0 0 0
g(x) 0 3a x 2(bx 2c)a b 0
g(x ) 0 3ax 2bx c 0
Vy iu kin l
2 2 2
0 02
0 0
3a x 2(bx 2c)a b 0
3ax 2bx c 0
phng trnh 3 2ax bx cx d 0 c 3 nghim th phng trnh g(x) 0 phi c 2 nghim phn bit v khng c nghim no trng vi
0x . Do ta c iu kin l
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vy iu kin cn tm l
2 2 2
0 02
0 0
3a x 2(bx 2c)a b 0
3ax 2bx c 0
Cch 2:cch lm thun gii tch
Xt hm s 3 2y ax bx cx d
Tp xc nh D
o hm 2y ' 3ax 2bx c
2y ' 0 g(x) 3ax 2bx c 0
phng trnh 3 2ax bx cx d 0 c 1 nghim th hm s 3 2y ax bx cx d khng c cc tr hoc c 2 cc tr cng nm
v mt pha ca Ox do ta c iu kin l
2
2
2 2 3 3 2 2cd ct
b 3ac 0' g(x) 0
' g(x) 0 b 3ac 0
y y 0 27a d 18abcd 4ac 4b d b c 0
Vy iu kin l
2
2
2 2 3 3 2 2
b 3ac 0
b 3ac 0
27a d 18abcd 4ac 4b d b c 0
phng trnh 3 2ax bx cx d 0 c 2 nghim th hm s 3 2y ax bx cx d c 2 cc tr v mt trong 2 gi tr cc tr phi
bng 0 do ta c iu kin l
2 2 2
0 02
0 0 0
g(x) 0 3a x 2(bx 2c)a b 0
g(x ) 0 3ax 2bx c 0
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22 2 3 3 2 2cd ct
' g(x) 0 b 3ac 0
y y 0 27a d 18abcd 4ac 4b d b c 0
Vy iu kin l
2
2 2 3 3 2 2
b 3ac 0
27a d 18abcd 4ac 4b d b c 0
phng trnh 3 2ax bx cx d 0 c 3 nghim phn bit th
hm s 3 2y ax bx cx d phi c 2 cc tr v 2 gi tr cc tr
tng ng tri du vi nhau do ta c iu kin l
2
2 2 3 3 2 2cd ct
' g(x) 0 b 3ac 0
y y 0 27a d 18abcd 4ac 4b d b c 0
Vy iu kin cn tm l
2
2 2 3 3 2 2
b 3ac 0
27a d 18abcd 4ac 4b d b c 0
Da vo tnh cht ny ta c th m rng ra
iu kin phng trnh bc 3 c 3 nghim dng l
2
2 2 3 3 2 2
b 3ac 0
27a d 18abcd 4ac 4b d b c 0
ad 0,ac 0,ab 0
iu kin phng trnh bc 3 c 3 nghim m l
2
2 2 3 3 2 2
b 3ac 0
27a d 18abcd 4ac 4b d b c 0
ad 0,ac 0,ab 0
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iu kin phng trnh bc 3 c 3 nghim trong c ng 2 nghim
dng v nghim cn li m l
2
2 2 3 3 2 2
b 3ac 0
27a d 18abcd 4ac 4b d b c 0
ad>0,ab0,ab>0
Trn Quang Minh K2008-2011 Thpt L T Trng Nha Trang Khnh Ha