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How do you isolate a variable in a function such as this

  1. Aug 19, 2009 #1
    x^4+y^4+z^4+(4x^2)*(y^2)*(z^2)-34=0
    ive tried all sorts of things factoring, algebraic manipulation ect. but i dont get how you could isolate a variable for a function like this i.e z= f(x,y).
     
  2. jcsd
  3. Aug 19, 2009 #2
    You may not be able express one variable explicitly in terms of the other two.
     
  4. Aug 19, 2009 #3
    Everything seems to be raised to powers of two. Therefore for some variable say x, replace x^2 with w, and then find w using the quadratic equation.
     
  5. Aug 19, 2009 #4

    HallsofIvy

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    If you want to solve for z (isolate z), treat it as a polynomial is z. For example, write it as [itex]z^4+ [4x^2y^2]z^2+ [x^4+ y^4- 34]= 0[/itex]
    That's a fourth degree equation but, as John Chrieto suggests, since it has only even powers of z, letting [itex]w= z^2[/itex] gives [itex]w^2+ [4x^2y^2]w+ [x^4+ y^4- 34]= 0[/itex], a quadratic equation that can be solved using the quadratic formula- giving two solutions in terms of x and y. Then you can take the two square roots of each of those to give the four solutions to the fourth degree equation.
     
  6. Aug 19, 2009 #5
    wow thank you guys you went well and beyond what i was thinking i didnt even think to substitute any of the squared variables and solve as a qudratic. Great insight/ intellegence.
     
  7. Aug 19, 2009 #6
    ok so i tried doing that but i get stuck at this point: i did w^2+w*u+v=0 where w=z^2 and u and v are both equal to the separated "clump" functions. then i plug the variables into the quadratic equation and substitute and i get radical (x^4*(4y^4-1)-y^2-34)-2x^2*y^2 i don't get how you get two functions one in x and one in y.
     
  8. Aug 20, 2009 #7

    HallsofIvy

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    Why do you think you should?

    Very few functions of 3 variables can be reduce to "z= f(x)g(y)".
     
  9. Aug 21, 2009 #8
    idk i just wasn't sure if i was doing something wrong or if i missed a step, also is your not the only one to say that im curious is it just a general assumption or is there a proof for different classes of functions that tells if they can be reduced to z=f(x)y(x) ?
     
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