Thu gọn các đa thức
a)\(2a^2x^3-ax^3-a^4-a^2x^3-ax^3+2a^4\) b)\(3xx^4+4xx^3-5x^2x^3-5x^2x^2\)
c)\(3a.4b^2-0,8b.4b^2-2ab.3b+b.3b^2-1\) d)\(5x.2y^2-5x.3xy-x^2y+6xy^2\)
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Bài làm
a) 2a²x³ - ax³ - a⁴ - x³a² - ax³ - 2x⁴
= 2a²x³ - ax³ - a⁴ - a²x³ - ax³ - 2x⁴
= ( 2a²x³ - a²x³ ) - ( ax³ + ax³ ) - a⁴ - 2ax⁴
= a²x³ - 2ax³ - a⁴ - 2ax⁴
b) 3xx⁴ + 4xx³ - 5x²x³ - 5x²x²
= 3x⁵ + 4x⁴ - 5x⁵ - 5x⁴
= ( 3x⁵ - 5x⁵ ) + ( 4x⁴ - 5x⁴ )
= -2x⁵ - x⁴
c) 3a - 4b² - 0,8b . 4b² - 2ab . 3b + b . 3b² - 1
= 3a - 4b² - 3,2b³ - 6ab² + 3b³ - 1
= 3a - 4b² - 0,2b³ - 6ab² - 1
d) 5x.2y² - 5x.3xy - x²y + 6xy²
= 10xy² - 15x²y - x²y + 6xy²
= ( 10xy² + 6xy² ) - ( 15x²y + x²y )
= 16xy² - 16x²y
\(\left(2x+1\right)^2-2\left(2x+1\right)\left(3-x\right)+\left(x-3\right)^2\)
\(=\left(2x+1\right)^2+2\left(2x-1\right)\left(x-3\right)+\left(x-3\right)^2\)
\(=\left(2x+1+x-3\right)^2\)
\(=\left(3x-2\right)^2\)
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\(a^3+3a^2-6a-8\)
\(=a^3+4a^2-a^2-4a-2a-8\)
\(=\left(a^3+4a^2\right)-\left(a^2+4a\right)-\left(2a+8\right)\)
\(=a^2\left(a+4\right)-a\left(a+4\right)-2\left(a+4\right)\)
\(=\left(a+4\right)\left(a^2-a-2\right)\)
\(=\left(a+4\right)\left(a^2-2a+a-2\right)\)
\(=\left(a+4\right)\left[\left(a^2-2a\right)+\left(a-2\right)\right]\)
\(=\left(a+4\right)\left[a\left(a-2\right)+\left(a-2\right)\right]\)
\(=\left(a+4\right)\left(a-2\right)\left(a+1\right)\)
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\(2x^2-5x+2\)
\(=2x^2-4x-x+2\)
\(=\left(2x^2-4x\right)-\left(x-2\right)\)
\(=2x\left(x-2\right)-\left(x-2\right)\)
\(=\left(x-2\right)\left(2x-1\right)\)
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\(x^2-2x-4y^2-4y\)
\(=\left(x^2-4y^2\right)-\left(2x-4y\right)\)
\(=\left(x-2y\right)\left(x+2y\right)-2\left(x-2y\right)\)
\(=\left(x-2y\right)\left(x+2y-2\right)\)
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\(a^2-1+4b-4b^2\)
\(=a^2-\left(1-4b+4b^2\right)\)
\(=a^2-\left(1-2b\right)^2\)
\(=\left(a-1+2b\right)\left(a+1-2b\right)\)
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\(a^4+6a^2b+9b^2-1\)
\(=\left(a^4+6a^2b+9b^2\right)-1\)
\(=\left(a^2+3b\right)^2-1\)
\(=\left(a^2+3b-1\right)\left(a^2+3b+1\right)\)
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\(2x^3+16y^3\)
\(=2\left(x^3+8y^3\right)\)
\(=2\left(x+2y\right)\left(x^2-2xy+4y^2\right)\)
Lần sau ghi đề tách riêng từng câu ra nhé em. Ghi dính chùm vậy khó nhìn lắm. Sẽ ít ai giải cho em
(3x^3 - 2x^2 + x + 2)(5x^2)
= 15x^5 - 10x^4 + 5x^3 + 10x^2
(3x^2 + 5x - 2)(2x^2 - 4x + 3)
= 3x^4 - 12x^3 + 9x^2 + 10x^3 - 20x^2 + 15x - 4x^2 + 8x - 6
= 6x^4 - 2x^3 - 15x^2 + 23x - 6
a) 4x^2 - 12xy + 9y^2
=(2x)^2 - 2.2.3xy + (3y)^2
=(2x+3y)^2
b) 27a^3 - 64b^3
=(3a)^3 - (4b)^3
=(3a - 4b) [(3a)^2 +3a.4b +(4B)^2]
d) (2x - 6y)^2 - (3xy - 4)^2
=[ (2x - 6y)+ (3xy - 4) ] [ (2x - 6y)- (3xy - 4) ]
\(1,a,4x^2-12xy+9y^2\)
\(=\left(2x\right)^2-2.3.2xy+\left(3y\right)^2\)
\(=\left(2x-3y\right)^2\)
\(b,27a^3-64b^3\)
\(=\left(3a\right)^3-\left(4b\right)^3\)
\(\left(3a-4b\right)\left(9a^2+12ab+16b^2\right)\)
`#3107`
`a)`
`A=`\(3x^4 + \dfrac{1}3xyz - 3x^4 - \dfrac{4}3xyz + 2x^2y - 6z\)
`= (3x^4 - 3x^4) + (1/3xyz - 4/3xyz) + 2x^2y - 6z`
`= -xyz + 2x^2y - 6z`
Thay `x = 1; y = 3` và `z = 1/3` vào A
`A = -1*3*1/3 + 2*1^2*3 - 6*1/3`
`= -1 + 6 - 2`
`= 6 - 3`
`= 3`
Vậy, `A=3`
`b)`
`B=`\(4x^3 - \dfrac{2}7xyz - 4x^3 - \dfrac{4}3xyz + 4x^2y\)
`= (4x^3 - 4x^3) + (-2/7xyz - 4/3xyz) + 4x^2y`
`= -34/21 xyz + 4x^2y`
Thay `x = -1; y = 2` và `z = -1/2` vào B
`B = -34/21*(-1)*2*(-1/2) + 4*(-1)^2 * 2`
`= -34/21 + 8`
`= 134/21`
Vậy, `B = 134/21`
`c)`
`C=`\(4x^2 + \dfrac{1}2xyz - \dfrac{2}3xy^2z - 5x^2yz + \dfrac{3}4xyz\)
`= 4x^2 + (1/2xyz + 3/4xyz) - 2/3xy^2z - 5x^2yz `
`= 4x^2 + 5/4xyz - 2/3xy^2z - 5x^2yz`
Ta có:
`|y| = 2`
`=> y = +-2`
Thay `x = -1; y = 2` và `z = 1/2` vào C
`4*(-1)^2 + 5/4*(-1)*2*1/2 - 2/3*(-1)*2^2*1/2 - 5*(-1)^2*2*1/2`
`= 4 - 5/4 + 4/3 - 5`
`= -11/12`
Vậy, với `x = -1; y = 2; z = 1/2` thì `B = -11/12`
Thay `x = -1; y = -2; z = 1/2`
`B = 4*(-1)^2 + 5/4*(-1)*(-2)*1/2 - 2/3*(-1)*(-2)^2*1/2 - 5*(-1)^2*(-2)*1/2`
`= 4 + 5/4 + 4/3 + 5`
`= 139/12`
Vậy, với `x = -1; y = -2; z = 1/2` thì `B = 139/12.`