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Exponential variables

  • Thread starter Chiborino
  • Start date
  • #1
21
0
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.
 

Answers and Replies

  • #2
Dick
Science Advisor
Homework Helper
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618
If you put u=e^x then e^(-x)=1/u. So it becomes quadratic equation in u. Is that enough of a hint?
 
  • #3
21
0
I'm not entirely sure I see how u+1/u can be plugged into the quadratic formula.

Could you please elaborate?
 
  • #4
Dick
Science Advisor
Homework Helper
26,258
618
u+1/u=4. Multiply both sides by u.
 
  • #5
21
0
Oh... I feel kind of dumb now. >_>
Thanks much.
 

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