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a: \(\dfrac{1}{2}x^2\cdot2x^3-4x^2+3=x^5-4x^2+3\)
b: \(2y\left(xy-1\right)\left(xy+1\right)=2y\left(x^2y^2-1\right)=2x^2y^3-2y\)
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a: \(\left(\dfrac{1}{3}x+2y\right)\left(\dfrac{1}{9}x^2-\dfrac{2}{3}xy+4y^2\right)=\dfrac{1}{27}x^3+8y^3\)
b: \(\left(x^2-\dfrac{1}{3}\right)\left(x^4+\dfrac{1}{3}x^2+\dfrac{1}{9}\right)=x^6-\dfrac{1}{27}\)
c: \(\left(y-5\right)\left(y^2+5y+25\right)=y^3-125\)
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\(\left(\dfrac{1}{3}x+2y\right)\left(\dfrac{1}{9}x^2-\dfrac{2}{3}xy+4y^2\right)=\dfrac{1}{27}x^3+8y^3\)
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\(\left(\dfrac{1}{3}x+2y\right).\left(\dfrac{1}{9}x^2-\dfrac{2}{3}xy+4y^2\right)=\left(\dfrac{1}{3}x+2y\right).[\left(\dfrac{1}{3}x\right)^2-\dfrac{2}{3}xy+\left(2y\right)^2] \)
= \(\left(\dfrac{1}{3}x+2y\right)^3\)
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a) Ta có: \(\left(x+2y\right)\left(x^2-2xy+4y^2\right)-\left(x-y\right)\left(x^2+xy+y^2\right)\)
\(=x^3+\left(2y\right)^3-\left(x^3-y^3\right)\)
\(=x^3+8y^3-x^3+y^3\)
\(=9y^3\)
b) Ta có: \(\left(x+1\right)\left(x-1\right)^2-\left(x+2\right)\left(x^2-2x+4\right)\)
\(=\left(x+1\right)\left(x^2-2x+1\right)-\left(x+2\right)\left(x^2-2x+4\right)\)
\(=x^3-2x^2+x+x^2-2x+1-\left(x^3+8\right)\)
\(=x^3-x^2-x+1-x^3-8\)
\(=-x^2-x-7\)
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a) 2x.(x2 - 7x - 3)
= 2xx2 + 2x(-7x) + 2x(-3)
= 2x2x - 2.7xx - 2.3x
= 2x3 - 14x2 - 6x
Ta có x3+8y3+x2+4y2-5 => x3+ (2y)3+x2+4(-5)+(2y)2+ 3.5 = x3+ (2y)3+x2+4(-5)+(2y)2+15( 1)
Thay xy = -5 vào 1, ta có:
x3+ (2y)3+x2+4xy+(2y)2+15 = (x+2y)[x2-x2y + (2y)2] + (x+2y)2 + 15 = (x+2y)[x2+2.x.2y +(2y)2-6xy] + (x+2y)2 + 15 = (x+2y)[ (x+2y)2 - 6xy] + 15 (2)
Thay x+2y = -2 và xy=-5 vào 2, ta có:
-2[ (-2)2 - 6.(-5)] + 15 = -2 ( 4 + 30) + 15 = -2. 34 + 15 = -53
Vậy khi x+2y = 2 và xy = -5 => x3+8y3+x2+4y2-5 = -53