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Ta có x3 + y3 + z3 - 3xyz=\(\left(x+y\right)^3-3x^2y-3xy^2+z^3-3xyz\)
\(=\left(x+y\right)^3+z^3-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left(\left(x+y\right)^2-z\left(x+y\right)+z^2\right)-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left(x^2+2xy+y^2-zx-yz-3xy\right)=0\)
Vì x3 + y3 + z3 - 3xyz=0 nên x3 + y3 + z3=3xyz
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x + y - z =0 --> x + y = z
Đặt : A = x3 + y3 - z3
Ta có : A= x3 + y3 - z3
A= ( x + y)3 - 3xy(x + y) - z3
A = ( x + y - z).[( x+y)2 + ( x+ y).z + z2] - 3xy(x+y)
Thay x + y = z vào A ta có :
A = ( z - z).( z2 + z.z + z2 ) - 3xyz
A = 0.( z2 + z.z + z2 ) - 3xyz
A= -3xyz ( đpcm )
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Ta có: \(x+y+z=0\Leftrightarrow x+y=-z\)
\(\Leftrightarrow\left(x+y\right)^3=\left(-z\right)^3\)
\(\Leftrightarrow x^3+3x^2y+3xy^2+y^3=-z^3\)
\(\Leftrightarrow x^3+y^3+z^3=-3x^2y-3xy^2\)
\(\Leftrightarrow x^3+y^3+z^3=-3xy\left(x+y\right)\)
\(\Leftrightarrow x^3+y^3+z^3=-3xy\left(-z\right)=3xyz\)(đpcm)
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= ( x3 + 3x2 y +3xy2 + y3 ) + z3 - 3 x 2 y - 3xy2 - 3xyz
= ( x + y ) 3 + z3 ] - 3xy x ( x + y + z )
= ( x + y + z ) x [ ( x + y ) 2 - z ( x + y ) + z2 ] - 3xy x ( x + y + z )
= ( x + y + z ) x ( x2 + 2xy + y2 + zx - zy + z2 - 3xy )
= ( x + y + z ) . ( x2 + y2 + z2 - xy - yz - zx )
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Xét \(A=x^3+y^3+z^3-3xyz\)
\(=\left(x^3+3x^2y+3xy^2+y^3\right)+z^3-3x^2y-3xy^2-3xyz\)
\(=\left(x+y\right)^3+z^3-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left(x^2+2xy+y^2-xz+yz+z^2\right)-3xy\left(x+y+z\right)\)
Với \(x+y+z=0\) thì \(A=0.\left(x^2+2xy+y^2-xz-yz+z^2\right)-3xy.0=0\)
\(A=x^3+y^3+z^3-3xyz=0\Rightarrow x^3+y^3+z^3=3xyz\) (đpcm)
x+y+z=0=> (x+y+z)(x2+y2+z2-xy-yz-zx)=0 (*)
Nhân (*) ra được :
x3+y3+z3-3xyz=0<=> x3+y3+z3= 3xyz(đpcm)
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\(x+y-z=0\)
\(\Leftrightarrow x+y=z\)
Lập phương 2 vế ta có:
\(\left(x+y\right)^3=z^3\)
\(\Leftrightarrow x^3+3x^2y+3xy^2+y^3=z^3\)
\(\Leftrightarrow x^3+y^3-z^3=-3x^2y-3xy^2\)
\(\Leftrightarrow x^3+y^3-z^3=-3xy\left(x+y\right)\)
Thay \(x+y=z\) vào biểu thức ta được
\(\Leftrightarrow x^3+y^3-z^3=-3xyz\)(đpcm)
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a,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)
b, Từ:
x + y + z = 0
=> x + y = -z
<=> (x + y)^3 = (-z)^3
<=> x^3 + 3x^2y + 3xy^2 + y^3 = -z^3
<=> x^3 + y^3 + z^3 = -3x^2y - 3xy^2
<=> x^3 + y^3 + z^3 = -3xy(x+y)
<=> x^3 + y^3 + z^3 = -3xy(-z)
<=> x^3 + y^3 + z^3 = 3xyz
Ta có: x3 + y3 + z3 = (x + y)(x2 - xy + y2) + z3 = (x + y)3 + z3 - 3xy(x + y)
= (x + y + z)[(x + y)2 - z(x + y) + z2) - 3xy(x + y + z) + 3xyz
= 3xyz (vì x + y + z = 0)
=> đpcm
cho x +y + z = 0. CMR : x3 + y3 + z3 = 3xyz
có : x + y +z =0 => x + y = -z
( x + y )3 = -z3
x3 + 3x2y + 3xy2 + y3 = -z3
=> x3 + y3 + 3xy( x+y ) = -z3
x3 + y3 + 3xy. ( -z) = -z3
x3 + y3 -3xyz = -z3
x3 + y3 + z3 = 3xyz ( đpcm )