Select one root of an second degree equation

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Pavel
Pavel 2013-6-1
i have solved a second degree equation in matlab using the roots() command and i want only one root to be displayed. How do i do this in MATLAB? Thank you very much

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采纳的回答

Roger Stafford
Roger Stafford 2013-6-1
You could also use the formula for second degree equations. If the equation is:
a*x^2 + b*x + c = 0
then get the positive root, (if there is just one,) with:
r = (-b + sign(a)*sqrt(b^2-4*a*c))/(2*a);
  2 个评论
Walter Roberson
Walter Roberson 2013-6-2
There could be multiple positive roots; the above would give the greater of the two.
Roger Stafford
Roger Stafford 2013-6-2
That's why I said "if there is just one [positive root]".

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更多回答（3 个）

Andrei Bobrov
Andrei Bobrov 2013-6-1
编辑：Andrei Bobrov 2013-6-1
p = [3 -6 -7];
r = roots(p);
out = r(r>0);
  2 个评论
Walter Roberson
Walter Roberson 2013-6-1
Caution: if there are imaginary roots, then only real() of the roots will be considered by the ">" operator. You might want to use
out = r(imag(r) == 0 & r > 0)
to select only positive real roots. If none exist then "out" will be empty.

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Walter Roberson
Walter Roberson 2013-6-1
r = roots(...);
r(1)
Did you care which root it displays?
Pavel
Pavel 2013-6-1
thanks all for replaing

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