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Mathematica Mathematica plots wrong

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  1. Jun 30, 2014 #1
    I said to Mathematica to plot a graph as this:

    Plot[Exp[-Sqrt[-x]] + Exp[Sqrt[-x]] - 2 == 0, {x, -10, 40}].

    You can see that there is square root of -x, which should mean that the function is defined only for x<=0. But no, Mathematica would plot a curve also for positive values of x. How is this possible at all?
     
  2. jcsd
  3. Jun 30, 2014 #2
    Note that when x > 0, we have Exp[-Sqrt[-x]] + Exp[Sqrt[-x]] = Exp[-iSqrt[x]] + Exp[iSqrt[x]] = 2Cos[Sqrt[x]], which is defined and a real number for all real values of x > 0. The latter equality is a result of Euler's formula.
     
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