Lời giải:

Ta có:

$x^3+y^3+z^3=(x+y)^3-3xy(x+y)+z^3=(-z)^3-3xy(-z)+z^3$
$=(-z)^3+3xyz+z^3=3xyz$
Khi đó:

$2(x^5+y^5+z^5)=2[(x^3+y^3+z^3)(x^2+y^2+z^2)-(x^3y^2+x^3z^2+y^3x^2+y^3z^2+z^3x^2+z^3y^2)]$

$=2[3xyz(x^2+y^2+z^2)-x^2y^2(x+y)-y^2z^2(y+z)-z^2x^2(z+x)]$

$=6xyz(x^2+y^2+z^2)-2[x^2y^2(-z)+y^2z^2(-x)+z^2x^2(-y)]$

$=6xyz(x^2+y^2+z^2)+2(x^2y^2z+y^2z^2x+x^2x^2y)$

$=6xyz(x^2+y^2+z^2)+2xyz(xy+yz+xz)$

$=6xyz(x^2+y^2+z^2)+xyz[(x+y+z)^2-(x^2+y^2+z^2)]$

$=6xyz(x^2+y^2+z^2)+xyz[0-(x^2+y^2+z^2)]$

$=6xyz(x^2+y^2+z^2)-xyz(x^2+y^2+z^2)=5xyz(x^2+y^2+z^2)$

Ta có đpcm.

Từ giả thiết: \(x+y+z=0\Rightarrow x+y=-z\)

\(\Leftrightarrow\left(x+y\right)^3=\left(-c\right)^3\)

\(\Leftrightarrow x^3+3x^2y+3xy^2+y^3=-c^3\)

\(\Leftrightarrow x^3+y^3+z^3=-3xy\left(x+y\right)\)

\(\Leftrightarrow x^3+y^3+z^3=3xyz\) (1)

Nhận cả 2 vế của (1) với \(x^2+y^2+z^2\) ta được:

\(3xyz\left(x^2+y^2+z^2\right)=\left(x^2+y^2+z^2\right)\left(x^3+y^3+z^3\right)=x^5+x^3\left(y^2+z^2\right)+y^5+y^3\left(x^2+z^2\right)+z^5+z^3\left(x^2+y^2\right)\left(2\right)\)Do x + y + z =0 \(\Rightarrow y+z=-x\Rightarrow\left(y+z\right)^2=x^2\Leftrightarrow y^2+z^2=x^2-2yz\)Tương tự ta có:

\(x^2+y^2=z^2-2xy;x^2+z^2=y^2-2xz\)

Thay vào (2) ta được:

\(3xyz\left(x^2+y^2+z^2\right)=x^5+y^5+z^5+x^3\left(x^2-2yz\right)+y^3\left(y^2-2xz\right)+z^3\left(z^2-2xy\right)\)\(=2\left(x^5+y^5+z^5\right)-2xyz\left(x^2+y^2+z^2\right)\)

\(\Leftrightarrow2\left(x^5+y^5+z^5\right)=5xyz\left(x^2+y^2+z^2\right)\left(đpcm\right)\)

2[x5x3-4x-9y-8z]x[4x-4x+6xy]=0

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Ta có: x + y + z = 0 <=> y + z = -x

(y+z)⁵ = (-x)⁵

y⁵ + z⁵ + 5y⁴z + 10y³z² + 10y²z³ + 5yz⁴ = -x⁵

y⁵ + z⁵ + 5y⁴z + 10y³z² + 10y²z³ + 5yz⁴ + x⁵ = 0

x⁵ + y⁵ + z⁵ +5xyz[ y³ + 2y²z + 2yz² + z³ ] = 0

x⁵ + y⁵ + z⁵ + 5xyz[(y+z)(y² -yz -z²)+ 2yz(x+z)] = 0

x⁵ + y⁵ + z⁵ +5xyz[(y+z)(y² +yz + z²)] = 0

2.(x⁵ + y⁵ + z⁵) + 5xyz(y+z)(y²+yz+z²) - (x⁵ + y⁵ + z⁵) = 0

2(x⁵ + y⁵ + z⁵) - 5xyz[(y²+2yz+z²)+y²+z²] = 0

2(x⁵ + y⁵ + z⁵) = 5xyz[(y+z)² + y² + z²]

2(x⁵ + y⁵ + z⁵) = 5xyz[(-x)² + y² + z²]

2(x⁵ + y⁵ + z⁵) = 5xyz(x² + y² + z²).

Ta có: x+y+z=0
=>x+y=-z =>(x+y)^5=-z^5
hay x^5+y^5+5(x^4y+xy^4+2x³y²+2x²y³+)=-z^5
<=>x^5+y^5+z^5+5xy(x³+y³+2x²y+2x²y)=0
<=>x5+y^5+z^5+5xy(x+y)(x²-xy+y²+2xy)=0
<=>x^5+y^5+z^5-5xyz(x²+xy+y²)=0
<=>x^5+y^5+z^5=5xyz(x²+xy+y²)
<=>2(x^5+y^5+z^5)=5xyz(2x²+2xy+2y²)
<=>2(x^5+y^5+z^5)=5xyz[x²+y²+(x+y)²]
<=>2(x^5+y^5+z^5)=5xyz(x³+y²+z²)

Từ x+y+z=0 => y+z=-x => (y+z)⁵=-x⁵

=> \(y^5+5y^4z+10y^2z^2+10y^2z^3+5yz^4+z^5=-x^5\)

\(\Rightarrow\left(x^5+y^5+z^5\right)+5yz\left(y^3+2y^2z+2yz^2+z^3\right)=0\)

\(\Rightarrow\left(x^5+y^5+z^5\right)+5yz\left[\left(y+z\right)\left(y^2-yz+x^2\right)\right]=0\)

\(\Rightarrow\left(x^5+y^5+z^5\right)+5yz\left(y+z\right)\left(y^2+yz+z^2\right)=0\)

\(\Rightarrow2\left(x^5+y^5+z^5\right)-5xyz\left[\left(y^2+2yz+z^2\right)+y^2+z^2\right]=0\)

\(\Rightarrow2\left(x^5+y^5+z^5\right)=5xyz\left[\left(y+z\right)^2+y^2+z^2\right]\) (đpcm)

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