Using the solve function to solve for x in a quadratic

Hi,

I'm having a few problems using the solve function to solve my quadratic equation.

It's quite basic but still causing problems for me.

This is my code:

syms x positive

B=13.69515; A=1.01; Delta=7.425 n=354.375; a0=-0.01660;

solve(a0*x^2-(B/A-Delta)*x +n,x);

When this is run it returns:

ans =

(75*2^(1/2)*346625777^(1/2))/8383 - 1548975/8383

which is fine but I need the single value answer, not that string type answer.

For example, if I say,

Q=(75*2^(1/2)*346625777^(1/2))/8383 - 1548975/8383

I get the answer I need which is 50.7873

How I get this single answer first time around without getting that string. I'm sure it is something simple but I'm a bit of a novice so I'm unsure.

Thanks for any help!

A=

Answer 1

[Walter Roberson]

double() will convert a symbolic number to a double precision value.

Answer 2

[Titus Edelhofer]

Hi,

why do you then compute symbolically?

x = roots([a0 -(B/A-Delta) n]);
xpos = x(x>0)

gives you 50.7873.

Titus

[Titus Edelhofer]

But if you want to use the symbolic computation:

y = solve(a0*x^2-(B/A-Delta)*x +n,x);

yNumeric = double(y)

Titus

Answer 3

[Damian Duffy]

Thank you both for your answers. It's definitely cleared it up for me. I'm very unfamiliar with the symbolic toolbox.

For a case in which the values for B,A,Delta and n change at every iteration, is it straightforward to set up using symbolic computation?

Walter,I saw an answer you provided on this:

syms x y z xin = input('x ='); yin = input('y ='); zsol = solve('z-y=x', z); disp(['z = ' char(zsol)]); znum = double(subs(zsol, {x, y}, {xin, yin})); disp(['z = ' num2str(znum)]);

Would a similar solution be applicable for my example?

Thanks again!