Problem - find xy+yz+xz = f(a,b,c) a=x^2 + x y + y^2\\ b=y^2 + y z + z^2\\ c=x^2 + x z + z^2\\ 2(ab+bc+ac-a^2-b^2-c^2)=\\2((x^2 + x y + y^2) (x^2 + x z + z^2) + (x^2 + x z + z^2) (y^2 + y z + z^2) +\\ (x^2 + x y + y^2) (y^2 + y z + z^2)) - (x^2 + x y + y^2)^2 - (x^2 + x z + z^2)^2 - (y^2 + y z + z^2)^2=\\ 3 (x^2 y^2 + 2 x^2 y z + x^2 z^2 + 2 x y^2 z + 2 x y z^2 + y^2 z^2)=\\ 3 (xy + y z + x z )^2\\ xy + y z + x z=\sqrt{\frac{2(ab+bc+ac-a^2-b^2-c^2)}{3}}\\ (x+y+z)^2=\frac{a+b+c-xy - y z - x z}{2}\\ x+y+z=\sqrt{\frac{a+b+c-\sqrt{\frac{2(ab+bc+ac-a^2-b^2-c^2)}{3}}}{2}}
Problem - find xy+yz+xz = f(a,b,c)
ResponderEliminara=x^2 + x y + y^2\\
b=y^2 + y z + z^2\\
c=x^2 + x z + z^2\\
2(ab+bc+ac-a^2-b^2-c^2)=\\2((x^2 + x y + y^2) (x^2 + x z + z^2) + (x^2 + x z + z^2) (y^2 + y z + z^2) +\\
(x^2 + x y + y^2) (y^2 + y z + z^2)) - (x^2 + x y + y^2)^2 - (x^2 + x z + z^2)^2 - (y^2 + y z + z^2)^2=\\
3 (x^2 y^2 + 2 x^2 y z + x^2 z^2 + 2 x y^2 z + 2 x y z^2 + y^2 z^2)=\\
3 (xy + y z + x z )^2\\
xy + y z + x z=\sqrt{\frac{2(ab+bc+ac-a^2-b^2-c^2)}{3}}\\
(x+y+z)^2=\frac{a+b+c-xy - y z - x z}{2}\\
x+y+z=\sqrt{\frac{a+b+c-\sqrt{\frac{2(ab+bc+ac-a^2-b^2-c^2)}{3}}}{2}}
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