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Câu 1:
\(\dfrac{x^2-10x+21}{x^3-7x^2+x-7}=\dfrac{\left(x-7\right)\left(x-3\right)}{\left(x-7\right)\left(x^2+1\right)}=\dfrac{x-3}{x^2+1}\)
\(\dfrac{2x^2-x-15}{2x^3+5x^2+2x+5}=\dfrac{2x^2-6x+5x-15}{\left(2x+5\right)\left(x^2+1\right)}=\dfrac{\left(2x+5\right)\left(x-3\right)}{\left(2x+5\right)\left(x^2+1\right)}=\dfrac{x-3}{x^2+1}\)
Do đó: \(\dfrac{x^2-10x+21}{x^3-7x^2+x-7}=\dfrac{2x^2-x-15}{2x^3+5x^2+2x+5}\)
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a) \(x^3-5x^2+8x-4=x^3-x^2-4x^2+4x+4x-4\)
\(=x^2\left(x-1\right)-4x\left(x-1\right)+4\left(x-1\right)\)
\(=\left(x-1\right)\left(x^2-4x+4\right)=\left(x-1\right)\left(x-2\right)^2\)
b) \(x^3-3x+2=x^3+2x^2-2x^2-4x+x+2\)
\(=x^2\left(x+2\right)-2x\left(x+2\right)+\left(x+2\right)\)
\(=\left(x+2\right)\left(x^2-2x+1\right)=\left(x+2\right)\left(x-1\right)^2\)
c) \(x^3-5x^2+3x+9=x^3+x^2-6x^2-6x+9x+9\)
\(=x^2\left(x+1\right)-6x\left(x+1\right)+9\left(x+1\right)\)
\(=\left(x+1\right)\left(x^2-6x+9\right)=\left(x+1\right)\left(x-3\right)^2\)
d) \(x^3+8x^2+17x+10=x^3+2x^2+6x^2+12x+5x+10\)
\(=x^2\left(x+2\right)+6x\left(x+2\right)+5\left(x+2\right)\)
\(=\left(x+2\right)\left(x^2+6x+5\right)=\left(x+2\right)\left(x+5\right)\left(x+1\right)\)
e) \(x^3+3x^2+6x+4=x^3+x^2+2x^2+2x+4x+4\)
\(=x^2\left(x+1\right)+2x\left(x+1\right)+4\left(x+1\right)\)
\(=\left(x+1\right)\left(x^2+2x+4\right)\)
f) \(x^3+3x^2+3x+2=x^3+2x^2+x^2+2x+x+2\)
\(=x^2\left(x+2\right)+x\left(x+2\right)+\left(x+2\right)\)
\(=\left(x+2\right)\left(x^2+x+1\right)\)
Bài 3: (SBT/24):
a. \(\dfrac{5x+3}{x-2}\)=\(\dfrac{5x^2+13x+6}{x^2-4}\)
(5x+3) . (x2-4) = 5x3-20x+3x3-12
(x-2) . (5x2+13x+6) = 5x3+13x2+6x-10x2-26x-12 = 5x3-20x+3x2-12
=> (5x+3) (x2-4) = (x-2) (5x2+13x+6)
Vậy \(\dfrac{5x+3}{x-2}\)=\(\dfrac{5x^2+13x+6}{x^2-4}\)(đẳng thức đúng)
b. \(\dfrac{x+1}{x+3}\)=\(\dfrac{x^2+3}{x^2+6x+9}\)
(x+1) . (x2+6x+9) = x3+6x2+9x+x2+6x+9 = x3+7x2+15x+9
(x+3) . (x2+3) = x3+3x+3x2+9
=> (x+1) (x2+6x+9) ≠ (x+3) (x2+3)
Vậy \(\dfrac{x+1}{x+3}\)≠\(\dfrac{x^2+3}{x^2+6x+9}\)(đẳng thức sai)
Chữa lại: \(\dfrac{x+1}{x+3}\)=\(\dfrac{x^2+3}{x^{2_{ }}+6x+9}\)
c. \(\dfrac{x^2-2}{x^2-1}\)=\(\dfrac{x+2}{x+1}\)
(x2-2) . (x+1) = x3+x2-2x-2
(x2-1) . (x+2) = x3+2x2-x-2
=> (x2-2) (x+1) ≠ (x2-1) (x+2)
Vậy \(\dfrac{x^2-2}{x^2-1}\)≠\(\dfrac{x+2}{x+1}\)(đẳng thức sai)
Chữa lại: \(\dfrac{x^2+x-2}{x^2-1}\)=\(\dfrac{x+2}{x+1}\)
d. \(\dfrac{2x^2-5x+3}{x^2+3x-4}\)=\(\dfrac{2x^2-x-3}{x^2+5x+4}\)
(2x2-5x+3) . (x2+5x+4) = 2x4+10x3+8x2-5x3-25x2-20x+3x2+15x+12
= 2x4+5x3-14x2-5x+12
(x2+3x-4) . (2x2-x-3) = 2x4-x3-3x2+6x3-3x2-9x-8x2+4x+12
= 2x4+5x3-14x2-5x+12
=> (2x2-5x+3) (x2+5x+4) = (x2+3x-4) (2x2-x-3)
Vậy \(\dfrac{2x^2-5x+3}{x^2+3x-4}\)=\(\dfrac{2x^2-x-3}{x^2+5x+4}\)
\(a.\left(x+1\right)\left(x+3\right)\left(x+5\right)\left(x+7\right)+15\)
\(=\left(x+1\right)\left(x+7\right).\left(x+3\right)\left(x+5\right)+15\)
\(=\left(x^2+8x+7\right)\left(x^2+8x+15\right)+15\)
\(=\left(x^2+8x+11-4\right)\left(x^2+8x+11+4\right)+15\)
\(=\left(x^2+8x+11\right)^2-4^2+15\)
\(=\left(x^2+8x+11\right)-1\)
\(=\left(x^2+8x+11-1\right)\left(x^2+8x+11+1\right)\)
\(=\left(x^2+8x+10\right)\left(x^2+8x+12\right)\)
\(=\left(x^2+8x+10\right)\left(x^2+2x+6x+12\right)\)
\(=\left(x^2+8x+10\right)\left[x\left(x+2\right)+6\left(x+2\right)\right]\)
\(=\left(x+2\right)\left(x+6\right)\left(x^2+8x+10\right)\)
\(b.x^3+4x^2-5x=x^3-x^2+5x^2-5x=x^2\left(x-1\right)+5x\left(x-1\right)=\left(x-1\right)\left(x^2+5x\right)=x\left(x-1\right)\left(x+5\right)\)
\(c.x^3-5x^2+8x-4=x^3-x^2-4x^2+4x+4x-4=x^2\left(x-1\right)-4x\left(x-1\right)+4\left(x-1\right)=\left(x-1\right)\left(x^2-4x+4\right)=\left(x-1\right)\left(x-2\right)^2\)
a) Đặt \(A=\left(x+1\right)\left(x+3\right)\left(x+5\right)\left(x+7\right)+15=\left(\left(x+1\right)\left(x+7\right)\right)\cdot\left(\left(x+3\right)\left(x+5\right)\right)+15=\left(x^2+8x+7\right)\left(x^2+8x+15\right)+15\)
Đặt \(t=x^2+8x\), A trở thành:
\(\left(t+7\right)\left(t+15\right)+15=t^2+22t+120=\left(t+10\right)\left(t+12\right)\)
\(\Rightarrow A=\left(x^2+8x+10\right)\left(x^2+8x+12\right)=\left(x+4-\sqrt{6}\right)\left(x+4+\sqrt{6}\right)\left(x+2\right)\left(x+6\right)\)
Kl: \(A=\left(x+4-\sqrt{6}\right)\left(x+4+\sqrt{6}\right)\left(x+2\right)\left(x+6\right)\)
b) \(x^3+4x^2-5x=x\left(x^2+4x-5\right)=x\left(x-1\right)\left(x+5\right)\)
c) \(x^3-5x^2+8x-4=\left(x-1\right)\left(x^2-4x+4\right)=\left(x-1\right)\left(x-2\right)^2\)
x 2 − 6 x + 9 = x − 3 2
Nhân tử phụ của phân thức 5 x − 3 là x + 5 nên ta có