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Tuesday, November 17, 2015

2015/105) if a=xy^2+yz^2+zx^2 and b=x^2y+y^2z+z^2x then express (x^3-y^3)(y^3-z^3)(z^3-x^3) in terms of a and b

(x^3-y^3)(y^3-z^3)(z^3-x^3)
= (x^3-y^3)(y^3z^3-y^3x^3-z^6+z^3x^3)
=x^3y^3z^3-y^3x^6-z^6x^3+z^3x^6-y^6z^3+y^6x^3+y^3z^6-x^3z^3x^3
-y^3x^6-z^6x^3+z^3x^6-y^6z^3+y^6x^3+y^3z^6
= (z^3x^6+y^3 z^6 + x^3y^6) - (y^3x^6 + x^3z^6 + y^6z^3)
HENCE
(x^3-y^3)(y^3-z^3)^(z^3-x^3)
  = ((xy^2)^3 + (yz^2)^3+(zx^2)^3) -  ((x^2y)^3 +(y^2z)^3 + (z^2x)^3)\cdots(1)

we have
a^3+b^3 + c^3 = (a+b+c)^3 - 3(a+b)(b+c)(c+a)
so  ((xy^2)^3 + (yz^2)^3+(zx^2)^3) = (xy^2+yz^2+zx^2)^3- 3(xy^2+yz^2)(yz^2+zx^2)(zx^2+xy^2)
= (xy^2+yz^2+zx^2) - 3y(xy+z^2)z(x^2+yz)x(xz+y^2)
= b^2-3xyz(x^2+yz)(y^2+xz)(z^2+xy)\cdots(2)
similarly
((x^2y)^3 + (y^2z)^3+(z^2x)^3) =a^2-3xyz(x^2+yz)(y^2+xz)(z^2+xy)\cdots(3)
from (1), (2) and (3) we get

(x^3-y^3)(y^3-z^3)^(z^3-x^3))=b^3 - a^3



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