Chia Da Thuc Mot Bien Da Sap Xep 8

KIEÅM TRA BAØI 1/CUÕ Phaùt bieåu qui taéc chia moät ña thöùc A cho moät ñôn thöùc B (trong tröôøng hôïp moãi haïng töû cuûa ña thöùc A chia heát cho B). 2/ Laøm tính chia: a. (2x5 + 3x2 – 4x3) : 2x2 b. (3x2y2 + 6x2y3 – 12xy) : 3xy 3/ Laøm tính nhaân: (x2 - 4x – 3).(2x2 – 5x + 1)

Thöù 5 ngaøy 1 thaùng 11 naêm 2007

1. Pheùp chia heát: Ví duï: Thöïc hieän pheùp chia ña thöùc 2x4 - 13x3 + 15x2 + 11x - 3 cho ña thöùc x2 - 4x - 3.

(2)

(1)

2x4 - 13x3 + 15x2 + 11x - 3 x2 - 4x - 3 – 2x4 - 8x3 2x2 - 5x+ 1 2 - 5x3 + 6x 21x2 + 11x - 3 – - 5x3 + 20x2 + x2 15x - 4x - 3 – x2 - 4x - 3 0

Baøi 48 sbt trang 8: Laøm tính chia: a/ (6x2 + 13x - 5) : (2x + 5) b/ (x3 - 3x2 + x - 3) : (x - 3)

6x2 + 13x - 5 2x + 5 – 6x2 + 15x 3x - 1 - 2x - 5 – - 2x - 5 0

x3 - 3x2 + x - 3 x - 3 – x3 - 3x2 x2 + x-3 1 – x-3 0

2. Pheùp chia coù dö: Ví Chuù yù: sgk/31 duï: Thöïc Vôùihieän A, B tuøy pheùp yù:chia ña thöùc 5x3 - 3x2 + 7 cho ña thöùc x2 + 1. - Toàn taïi duy nhaát Q, R sao cho A = B.Q + R (1) (2) - R = 0: ta coù pheùp chia heát; - R ≠ 0: ta coù pheùp chia coù dö. 5x3 - 3x2 +7 x2 + 1 – 5x3 + 5x 5x - 3 - 3x2 - 5x + 7 – - 3x2 -3 + 5x 10

Baøi 50 sbt trang 8:Saép xeáp caùc ña thöùc theo luõy thöøa giaûm daàn cuûa bieán roài laøm pheùp chia. (x4 + x2 - 2x3 - 11 + 13x) : (x2 - 2x + 3)

x4 - 2x3 + x2 + 13x - 11 x2 - 2x + 3 – x4 - 2x3 + 3x2 x2 - 2 - 2x2 + 13x - 11 – - 2x2 + 4x - 6 9x - 5

Baøi 67 sgk trang 31: Saép xeáp caùc ña thöùc theo luõy thöøa giaûm daàn cuûa bieán roài laøm pheùp chia. a/ (x3 - 7x + 3 - x2) : (x - 3) b/ (2x4 - 3x3 - 3x2 - 2 + 6x) : (x2 - 2) 3 4 2x x - -x23x - 37x - 3x + 23+ 6x x -- 32 x2 - 2 – 2x x3 4- 3x3 - 4x2 x2 + 2x - 12 - 3x + 2 3 1 2x - 3x - 7x +x +2 3+ 6x 2x -2 –– 2x 2 3 - 3x - 6x + 6x 2 - x x+ 3 -2 – -–x x+ 2 3 -2 0 0

Baøi 69 sgk trang 31: Cho hai ña thöùc A = 3x4 + x3 + 6x - 5 vaø B = x2 + 1. Tìm dö R trong pheùp chia A cho B roài vieát A döôùi daïng A = B.Q +R 4 3x + x3 + 6x - 5 x2 + – 3x4 + 3x2 3x21+ x - 3 x3- 3x2 + 6x - 5 – x3 + x -3x2 - 5x - 5 – 3x2 -3 5x - 2 Vaäy: 3x4 + x3 + 6x - 5 = (x2 + 1).(3x2 + x - 3) +

- Hoïc baøi vaø xem laïi hai pheùp chia. - Baøi taäp veà nhaø: 70 ñeán 74 sgk. - Tieát sau luyeän taäp.

Baøi 68 sgk trang 31: AÙp duïng haèng ñaúng thöùc ñaùng nhôù ñeå tính nhanh. a/ (x2 + 2xy + y2) : (x + y) b/ (125x3 + 1) : (5x + 1) c/ (x2 - 2xy + y2) : (y - x) a/ (x2 + 2xy + y2) : (x + y)= (x + y)2 : (x + y) x + = b/ (125x3 + 1) : (5x + 1) y = (5x + 1).(25x2 - 5x + 1) : (5x + 1) = 25x2 - 5x + 1 c/ (x2 - 2xy + y2) : (y - x) = (x - y)2 : - (x - y) = - (x – y) = y - x