III. PHAN PH 6I THC)I LUONG
2.Dau cija tam thiire bae ha
• GV cho HS quan sat hinh trong bang.
HI. Neu A < 0 em cd nhan xet gi ve dau cua f(x) va dau ciia a. H2. Ne'u A = 0 em cd nhan xet gi ve dau ciia f(x) va dau cua a. HI. Neu A > 0 em cd nhan xet gi ve dau ciia f(x) va dau ciia a. HI. Em cd nhan xet gi ve dau ciia f(x) va da'u cua a.
• GV neu dinh li
Chof(x) = ax'^ +bx + c (a^ 0), A= b^ - 4ac.
Ne'u A<Q thif(x) cimg ddu vdi he sd a, vdi mgi x G M .
Ne'u A = Q thi fix) ludn cimg ddu vdi he sd a, trie khi x = — 2a Ne'u A>0 thi f(x) cimg ddu vdi he sd a khi x < x^ hoac x > Xj, trdi ddu vdi he sd a khi x^ < x < X2 trong dd x^, x^ (x\ < X2) Id hai nghiem ciiaf(x).
Sau dd neu mot sd cau hdi sau:
HI. Xet da'u tam thirc / ( x ) = 2x^ - 2 x + 1 . H2. Xet da'u tam thuc f(x) = 2x^ - 7 2 x +1 H3. Xet dau tam thirc f(x) = 2x^ - 4x +1
• GV neu vi du 1 va vi du 2 va hudng din HS giai theo SGK. (Chii y la da ap dung dinh li d dau?)
• Thuc hien H1
Hoat ddng cua GV Cau hdi 1.
Hay tinh nghiem cua f(x)
Cau hdi 2.
Khi nao f(x) > 0
Cau hdi 3.
Hay lam cac cau con lai tuang tu.
Hoat ddng cua HS Ggi y tra Idi cau hdi 1
1 7 xi - - 1 , xj--- xi - - 1 , xj---
Ggi y tra Idi cau hdi 2 X G (-1 ; 4 ) -
2
Ggi y tra Idi cau hdi 3
GV hudng din HS lam.
2 a > 0 Vx G M, ax + bx + c> 0 <»
A < 0
Vx G M, ax + bx + c < 0 <=> a < 0 A < 0
• Neu vi du 3 va hudng dan HS giai theo cac cau hdi sau day:
HI. f(x) eo phai la tam thiic bae hai hay khdng? H2. Vdi m = 2, f(x) cd dang nhu the' nao?
H3. Khi f(x) la tam thiic bae hai, hay ap dung nhan xet tren di tim m.
Thuc hien H2
Hoat ddng cua GV Cau hdi 1.
Khi nao thi f(x) la nhi thiic (adsbygoogle = window.adsbygoogle || []).push({});
bae nha't. Hay xet dau ciia
f(x) trong trudng hgp nay.
Cau hdi 2.
Khi m 7^ 2, f(x) > 0 vdi mgi x
khi nao?.
Hoat ddng cua HS Ggi y tra Idi cau hdi 1
Khi m = 2, trudng hgp nay khdng
thoa man vi f(x) cd the am.
Ggi y tra Idi cau hdi 2
fa = 2 - m > 0 Vx, f(x) > 0 <=> o [ A = m - 1 < 0 fm < 2 <=> m < 1. [m < 1 DE KIEM TRA 10'
Cdu 1. Cho /(x) = - 3 x - 2 x - l . Hay chgn ket kuan diing trong ket luan sau
day:
( a ) / ( x ) > 0 Vx G R. (b) / ( x ) < 0 Vx G R.
(c) f(x) > 0 V X thudc mdt khoing nao dd. (d) f(x) < 0 V X thudc mdt khoang nao dd. 114
)? - 3 x + 2 Cdu 2. Cho bat phuong trinh — , < 0.
72x-3
Chgn ke't qui sai trong cac ket qua sau
(a) Tap nghiem ciia bat phuong trinh la (1 ; 2); (b) Tap nghiem cua bat phuong trinh la [1 ; 2];
3 (c) Tap nghiem cua bat phuong trinh la (— ; 2);
3 (d) Tap nghiem cua bat phuang trinh la [— ; 2);
Ddp dn:
C a u l : ( b ) ; Cau 2: (c).
TOM T A T BAI HOC