How to find only positive root of a polynomial
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How to find only positive root of a polynomial equation x^4+7*x^2-A=0 where A is varying from 1:.1:3. If rr is the positive real root, then find rr/(rr+1) for each case.
poly = [1 0 7 0 A];
R = roots(poly);
if isa(R,'complex') && (R<=0)
Please correct the code.
Matt J on 16 Apr 2013
Edited: Matt J on 16 Apr 2013
By the quadratic formula, the largest solution for x^2 is
x^2 = (-7+ sqrt(49+4*A))/2
For positive A, this will always be positive. You can then get a positive root for x by doing
x = sqrt( (-7+ sqrt(49+4*A))/2 )
I don't know if this satisfies your requirement "without solving a polynomial equation". It doesn't seem possible that you meant this literally (see my comment above).