How to find a real positive root of a bi-quadratic equation

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Atom 2013-4-15

评论： Joel Mathew 2016-9-19

Suppose a bi-quadratic equation x^4+4*x^2-7=0. How to find a positive real root of it.

solve(x^4+4*x^2-7=0, x, Real, Positive)

Is the above correct?

How to use fsolve here?

Matt J 2013-4-15

编辑：Matt J 2013-4-15

Why not just use ROOTS?

    >> roots([1 0 4 -7])
    ans =
      -0.6277 + 2.2764i
      -0.6277 - 2.2764i
       1.2554

Matt J 2013-4-15

OK, then

 sol=roots([1 0 4 -7]);
 sol=sol(imag(sol)==0 & sol>=0);

Joel Mathew 2016-9-19

Thank you this worked for me

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