Select one root of an second degree equation

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

Answer 1

[Roger Stafford]

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);

[Walter Roberson]

There could be multiple positive roots; the above would give the greater of the two.

[Roger Stafford]

That's why I said "if there is just one [positive root]".

Answer 2

[Andrei Bobrov]

p = [3 -6 -7];
r = roots(p);
out =  r(r>0);

[Andrei Bobrov]

about positive root

[Walter Roberson]

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.

Answer 3

[Walter Roberson]

r = roots(...);
r(1)

Did you care which root it displays?

[Pavel]

i want the positive root

Answer 4

[Pavel]

thanks all for replaing