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Exercise NO. 1 : Algebra, x+y+z=0

28Jan08

A few days ago I were stuck with one exercise which I can’t pushed off :] Meanwhile when I forgotten about it, friend of mine, Ecik, did it. It’s below with his solution.

Prove the following:

x^3 + y^3 + z^3 =3xyz

if x+y+z=0

Solution:

x^3+y^3+z^3=(x+y) \cdot (x^2-xy+y^2)+z^3= \newline =-z \cdot (x^2-xy+y^2) + z^3=xyz-z \cdot (x^2+y^2-z^2) = \newline = xyz - z \cdot (x^2+y^2-(x+y)^2) = xyz - z \cdot (x^2+y^2-x^2-2xy-y^2) \newline = xyz +2xyz=3xyz

I’m open on any suggestions or other solutions of yours.

If you’d like to see Ecik’s blog, here it is
http://braindarkside.wordpress.com/

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Filed under: Exercises, Identity, Mathematics   |  2 Comments
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2 Responses to “Exercise NO. 1 : Algebra, x+y+z=0”

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  1. 1 Ofir on September 10, 2008 said:

    You can also use the identity a^3+b^3+c^3-3abc=(a+b+c)(a^2+b^2+c^2-ab-bc-ca)

    Reply
  2. 2 masteranza on September 10, 2008 said:

    Very cool. Thank you Ofir

    Reply

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