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20 tháng 10 2023
Sửa đề: \(x^3+y^3+z^3-3xyz\)
\(=\left(x+y\right)^3+z^3-3xy\left(x+y\right)-3xyz\)
\(=\left(x+y+z\right)\left[\left(x+y\right)^2-z\left(x+y\right)+z^2\right]-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left(x^2+2xy+y^2-xz-yz+z^2-3xy\right)\)
\(=\left(x+y+z\right)\left(x^2+y^2+z^2-xy-xz-yz\right)\)
27 tháng 7 2016
Ta có:
x³ + y³ + z³ - 3xyz = (x+y)³ - 3xy(x-y) + z³ - 3xyz
= [(x+y)³ + z³] - 3xy(x+y+z)
= (x+y+z)³ - 3z(x+y)(x+y+z) - 3xy(x-y-z)
= (x+y+z)[(x+y+z)² - 3z(x+y) - 3xy]
= (x+y+z)(x² + y² + z² + 2xy + 2xz + 2yz - 3xz - 3yz - 3xy)
= (x+y+z)(x² + y² + z² - xy - xz - yz).
TH
5
13 tháng 12 2016
Ta có:
x³ + y³ + z³ - 3xyz = (x+y)³ - 3xy(x-y) + z³ - 3xyz
= [(x+y)³ + z³] - 3xy(x+y+z)
= (x+y+z)³ - 3z(x+y)(x+y+z) - 3xy(x-y-z)
= (x+y+z)[(x+y+z)² - 3z(x+y) - 3xy]
= (x+y+z)(x² + y² + z² + 2xy + 2xz + 2yz - 3xz - 3yz - 3xy)
= (x+y+z)(x² + y² + z² - xy - xz - yz).
~~~~~~~~
Bài làm trên mình đã sử dụng hằng đẳng thức đáng nhớ sau:
(a+b)³ = a³ + 3a²b + 3ab² + b³ = a³ + b³ + 3ab(a-b)
=> a³ + b³ = (a+b)³ - 3ab(a-b).
Chúc bạn học giỏi!
13 tháng 12 2016
x³ + y³ + z³ - 3xyz = (x + y)³ - 3xy(x + y) + z³ - 3xyz
= (x + y)³ + z³ - 3xy(x + y + z)
= (x + y + z)³ - 3(x + y + z)(x + y)z - 3xy(x + y + z)
= (x + y + z)³ - 3(x + y + z)(xy + yz + zx)
= (x + y + z)[(x + y + z)² - 3xy - 3yz - 3zx)]
= (x + y + z)(x² + y² + z² - xy - yz - zx)
NH
2
28 tháng 9 2015
\(x^3+y^3+z^3+3xyz\)
\(=\left(x+y\right)^3-3xy\left(x+y\right)+z^3+3xyz\)
\(=\left(x+y\right)^3-3xy\left(x+y+z\right)+z^3\)
\(=\left(x+y+z\right)^3-3\left(x+y\right)z\left(x+y+z\right)-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)^3-3\left(x+y+z\right)\left(xy+yz+xz\right)\)
\(=\left(x+y+z\right)\left[\left(x+y+z\right)^2-3xy-3yz-3xz\right]\)
\(\left(x+y+z\right)\left(x^2+y^2+z^2-xy-yz-xz\right)\)
VN
0
4 tháng 8 2016
\(x^3+y^3+z^3-3xyz\)
\(=\left(x+y\right)^3-3x^2y-3xy^2+z^3-3xyz\)
\(=\left[\left(x+y\right)^3+z^3\right]-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left[\left(x+y\right)^2-z\left(x+y\right)+z^2\right]-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left(x^2+y^2+2xy-xz-yz+z^2-3xyz\right)\)
\(=\left(x+y+z\right)\left(x^2+y^2+z^2-xz-yz-xy\right)\)
1 tháng 7 2015
= (x + y)3 + z3 – 3x2y – 3xy2 - 3xyz
= (x + y +z)[(x + y)2 – (x + y)z + z2)] - 3xy(x + y + z)
= (x + y + z)(x2 +2xy + y2 – xz – yz +z2 – 3xy)
= (x + y + z)(x2 + y2 +z2 – xy - yz – xz)
MT
1 tháng 7 2015
x3 - y3 - z3 +3xyz
= (x3 - 3x2y +3xy2 -y^3) +3x2y-3xy2 - z3 +3xyz
= [(x-y)3 -z3] + 3x2y -3xy2 +3xyz
= (x-y-z)(x2 + 2xy+y2 +zx+zy + z2) + 3xy( x-y+z)
x3−y3−z3+3xyz=(x+y+z)(xy+yz+xz−x2−y2−z2) =-(x^3+y^3+y^3-3xyz)$
Ta tính x3+y2+z3−3xyz trước
ta có:
x3+y3+z3−3xyz=(x+y)3+z3−3xy(x+y)−3xyz=(x+y+z)(x2+y2+z2−xy−yz−xz)
=>x3−y3−z3+3xyz=(x+y+z)(xy+zy+xz−x2−y2−z2)
H9
HT.Phong (9A5)
CTVHS
12 tháng 8 2023
\(\left(3x+1\right)^2-\left(3x-1\right)^2\)
\(=\left(3x+1-3x+1\right)\left(3x+1+3x-1\right)\)
\(=2\cdot6x\)
\(=12x\)
_________
\(\left(x+y\right)^2-\left(x-y\right)^2\)
\(=\left(x+y+x-y\right)\left(x+y-x+y\right)\)
\(=2x\cdot2y\)
\(=4xy\)
H9
HT.Phong (9A5)
CTVHS
12 tháng 8 2023
\(\left(x+y\right)^3+\left(x-y\right)^3\)
\(=\left(x+y+x-y\right)\left[\left(x+y\right)^2-\left(x+y\right)\left(x-y\right)+\left(x-y\right)^2\right]\)
\(=2x\cdot\left(x^2+2xy+y^2-x^2+y^2+x^2-2xy+y^2\right)\)
\(=2x\cdot\left(x^2+3y^2\right)\)
______
\(x^3+y^3+z^3-3xyz\)
\(=\left(x+y\right)^3-3xy\left(x-y\right)+z^3+3xyz\)
\(=\left[\left(x+y\right)^3+z^3\right]-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)^3-3z\left(x+y\right)\left(x+y+z\right)-3xy\left(x-y-z\right)\)
\(=\left(x+y+z\right)\left[\left(x+y+z\right)^2-3z\left(x+y\right)-3xy\right]\)
\(=\left(x+y+z\right)\left(x^2+y^2+z^2+2xy+2xz+2yz-3xz-3yz-3xy\right)\)
\(=\left(x+y+z\right)\left(x^2+y^2-xy-xz-yz\right)\)
20 tháng 2 2017
\(=\left(x^3+y^3\right)+z^3-3xyz=\left(x+y\right)^3+z^3-3xy\left(x+y\right)-3xyz\)
\(=\left(x+y+z\right)^3-3z\left(x+y\right)\left(x+y+z\right)-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left(\left(x+y+z\right)^2-3z\left(x+y\right)-3xy\right)\)
\(=\left(x+y+z\right)\left(x^2+y^2+z^2-xy+yz+zx\right)\)
\(=\left(x+y\right)^3-3xy\left(x+y\right)+z^3-3xyz\\ =\left(x+y+z\right)\left(x^2+2xy+y^2-xz-yz+z^2\right)-3xy\left(x+y+z\right)\\ =\left(x+y+z\right)\left(x^2+y^2+z^2-xy-yz-zx\right)\)
\(x^3+y^3+z^3-3xyz\)
\(=\left(x^3+3x^2y+3xy^2+y^3\right)+z^3-3x^2y-3xy^2-3xyz\)
\(=\left(x+y\right)^3+z^3-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left[\left(x+y\right)^2-\left(x+y\right)z+z^2\right]-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left(x^2+2xy+y^2-xz-yz+z^2-3xy\right)\)
\(=\left(x+y+z\right)\left(x^2+y^2+z^2-xy-yz-xz\right)\)