Cái nè k cần làm nhé

`#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.`

a: \(2x^2+3xy-14y^2\)

\(=2x^2+7xy-4xy-14y^2\)

\(=\left(2x^2+7xy\right)-\left(4xy+14y^2\right)\)

\(=x\left(2x+7y\right)-2y\left(2x+7y\right)\)

\(=\left(2x+7y\right)\left(x-2y\right)\)

b: \(\left(x-7\right)\left(x-5\right)\left(x-3\right)\left(x-1\right)+7\)

\(=\left(x-7\right)\left(x-1\right)\left(x-5\right)\left(x-3\right)+7\)

\(=\left(x^2-8x+7\right)\left(x^2-8x+15\right)+7\)

\(=\left(x^2-8x\right)^2+15\left(x^2-8x\right)+7\left(x^2-8x\right)+105+7\)

\(=\left(x^2-8x\right)^2+22\left(x^2-8x\right)+112\)

\(=\left(x^2-8x\right)^2+8\left(x^2-8x\right)+14\left(x^2-8x\right)+112\)

\(=\left(x^2-8x\right)\left(x^2-8x+8\right)+14\left(x^2-8x+8\right)\)

\(=\left(x^2-8x+8\right)\left(x^2-8x+14\right)\)

c: \(\left(x-3\right)^2+\left(x-3\right)\left(3x-1\right)-2\left(3x-1\right)^2\)

\(=\left(x-3\right)^2+2\left(x-3\right)\left(3x-1\right)-\left(x-3\right)\left(3x-1\right)-2\left(3x-1\right)^2\)

\(=\left(x-3\right)\left[\left(x-3\right)+2\left(3x-1\right)\right]-\left(3x-1\right)\left[\left(x-3\right)+2\left(3x-1\right)\right]\)

\(=\left(x-3+6x-2\right)\left(x-3-3x+1\right)\)

\(=\left(7x-5\right)\left(-2x-2\right)\)

\(=-2\left(x+1\right)\left(7x-5\right)\)

d: \(xy\left(x-y\right)+yz\left(y-z\right)+zx\left(z-x\right)\)

\(=x^2y-xy^2+y^2z-yz^2+zx\left(z-x\right)\)

\(=\left(x^2y-yz^2\right)-\left(xy^2-y^2z\right)+xz\left(z-x\right)\)

\(=y\left(x^2-z^2\right)-y^2\left(x-z\right)-xz\left(x-z\right)\)

\(=y\cdot\left(x-z\right)\left(x+z\right)-\left(x-z\right)\left(y^2+xz\right)\)

\(=\left(x-z\right)\left(xy+zy-y^2-xz\right)\)

\(=\left(x-z\right)\left[\left(xy-y^2\right)+\left(zy-zx\right)\right]\)

\(=\left(x-z\right)\left[y\cdot\left(x-y\right)-z\left(x-y\right)\right]\)

\(=\left(x-z\right)\left(x-y\right)\left(y-z\right)\)

1) 2x²-8xy-5x+20y

=2x(x-4y)-5(x-4y)

=(2x-5)(x-4y)

2) x³-x²y-xy+y²

=x²(x-y)-y(x-y)

=(x²-y)(x-y)

3) x²-2xy-4z²+y²

=(x-y)²-(2z)²

=(x-y-2z)(x-y+2z)

4) a³+a²b-a²c-abc

=a²(a+b)-ac(a+b)

=(a²-ac)(a+b)

=a(a-c)(a+b)

5) x³+y³+3x²y+3xy²-x-y

=(x+y)(x²-xy+y²)+3xy(x+y)-(x+y)

=(x+y)(x²-xy+y²+3xy-1)

=(x+y)[(x+y)²-1)]

=(x+y)(x+y+1)(x+y-1)

6) x³+x²y-x²z-xyz

=x²(x+y)-xz(x+y)

=(x²-xz)(x+y)

=x(x-z)(x+y)

7) =[x(y+z)²-2xyz]+[y(z+x)²-2xyz]+z(x+y)²

=x(y²+z²)+y(z²+x²)+z(x+y)²

=xy(x+y)+z²(x+y)+z(x+y)²

=(x+y)(xy+z²+zx+zy)

=(x+y)(x+z)(y+z)

8) x³(z-y)+y³(x-z)+z³(y-x)

Tách x-z= -[z-y+y-x]

a) Áp dụng BĐT AM-GM cho 3 số: \(x^3+y^3+z^3\ge3xyz\) dấu "=" xảy ra \(\Leftrightarrow x=y=z\)

Mà đề cho \(x^3+y^3+z^3=3xyz\)nên ta được \(x=y=z\)

\(\Rightarrow\frac{7x^3+y^3+12z^3}{2x^2y+3xyz+5xz^2}=\frac{7x^3+x^3+12z^3}{2x^3+3x^3+5x^3}=\frac{20x^3}{10x^3}=2\)

b) Áp dụng BĐT AM-GM cho 4 số dương: \(x^4+y^4+z^4+t^4\ge4xyzt\)

Mà đề cho dấu "=" xảy ra vậy đề bài tương đương với \(x=y=z=t\)

\(\frac{x^6+2y^6+3z^6+4xyz^4+10yzt^4}{5xy^2z^3}=\frac{x^6+2x^6+3x^6+4x^6+10x^6}{5x^6}=\frac{20x^6}{5x^6}=4\)

1, 2x² - 8xy - 5x + 20y

= (2x² - 5x) - (8xy - 20y)

= x(2x - 5) - 4y(2x - 5)

= (2x - 5) (x - 4y)

2,  x³ - x²y - xy + y²

= (x³ - xy) - (x²y - y²)

= x(x² - y) - y(x² - y)

= (x² - y) (x - y)

3, x² - 2xy - 4z² + y²

= (x² - 2xy + y²) - 4z²

= (x - y)² - (2z)² 

= (x - y - 2z) (x - y + 2z)

4, a³ + a²b - a²c - abc

= (a³ - a²c) + (a²b - abc)

= a²(a - c) + ab(a - c)

= (a - c) (a² + ab)

5, x³ + y³ + 3x²y + 3xy² - x - y

= (x³ + 3x²y + 3xy² + y³) - (x + y)

= (x + y) ³ - (x + y)

= (x + y) [(x + y)² - 1]

= (x + y) (x + y - 1) (x + y + 1)

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