a+b+c = 0 => a+b = -c
Ta có:
[TEX]a^3 + b^3 +c^3=[/TEX][TEX]a^3+b^3 -(a+b)^3 = a^3 + b^3 - a^3-b^3 - 3ab(a+b) = 3abc[/TEX]
\Rightarrow [TEX](a^3 + b^3 +c^3)(a^2 + b^2 +c^2) =[/TEX][TEX]3abc(a^2 + b^2 +c^2) [/TEX]
\Rightarrow [TEX]a^5 +b^5+c^5 + a^2b^2(a+b) + a^2c^2(a+c) + b^2c^2(b+c) = 3abc(a^2 + b^2 +c^2) [/TEX]
\Rightarrow [TEX]a^5 +b^5+c^5 - c.a^2b^2 -b.a^2c^2 -a.b^2c^2 = 3abc(a^2 + b^2 +c^2) [/TEX]
\Rightarrow [TEX]a^5 +b^5+c^5 - abc(ab+ac+bc) = 3abc(a^2 + b^2 +c^2) [/TEX] (*)
do a+b+c = 0 \Rightarrow [TEX]-(a^2 +b^2 +c^2)/2 = (ab+ac+bc)[/TEX] thay vào (*):
[TEX]a^5 +b^5+c^5 + abc(a^2 +b^2 +c^2)/2 = 3abc(a^2 + b^2 +c^2)[/TEX]
\Rightarrow [TEX]2(a^5 +b^5+c^5) = 6abc(a^2 + b^2 +c^2) - abc(a^2 +b^2 +c^2) = 5abc(a^2+b^2+c^2)[/TEX]