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Homework Help: Exponential variables

  1. Mar 9, 2010 #1
    My friend just came to me with this problem:
    e^x + e^-x = 4
    I aced precalc, and I still have no idea where to start on this one. I've factored, natural logged, factored and then natural logged... and I keep running into dead ends.

    My graphing calculator puts the answer at +/-1.3169 or something like that.

    It's not at all urgent, I'm just curious since I feel that I should know how to solve this problem.
     
  2. jcsd
  3. Mar 9, 2010 #2

    Dick

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    If you put u=e^x then e^(-x)=1/u. So it becomes quadratic equation in u. Is that enough of a hint?
     
  4. Mar 9, 2010 #3
    I'm not entirely sure I see how u+1/u can be plugged into the quadratic formula.

    Could you please elaborate?
     
  5. Mar 9, 2010 #4

    Dick

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    u+1/u=4. Multiply both sides by u.
     
  6. Mar 9, 2010 #5
    Oh... I feel kind of dumb now. >_>
    Thanks much.
     
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