# solve equation for a variable

10 views (last 30 days)
SETH SCHREINER on 28 Feb 2023
Commented: Anton Kogios on 28 Feb 2023
I'm sure this sort of thing has been posted numerous times, but I can't find anything that works for me for whatever reason. I need to solve an equation for a variable, and I can't remember how to do this. It's been a while since I've used matlab. I need to solve for t numerically. This is the last thing I tried:
v = 2.42;
si_f235 = 577E-24;
N_235 = 780.63E18;
phi = 3E13;
si_a235 = 101E-24;
si_a238 = 2.73E-24;
N_238 = 21.52E21;
n = 1.5;
eqn = @(x) v*si_f235*N_235(1-si_f235*phi*x)/(si_a235*N_235*(1-si_a235*phi*x)+si_a238*N_238*(1-si_a238*phi*x))==n;
t = vpasolve(eqn , x)
it's just telling me that x is an unrecognized variable

Anton Kogios on 28 Feb 2023
This works for me (you were also missing an * in your equation):
v = 2.42;
si_f235 = 577E-24;
N_235 = 780.63E18;
phi = 3E13;
si_a235 = 101E-24;
si_a238 = 2.73E-24;
N_238 = 21.52E21;
n = 1.5;
syms x
eqn = v*si_f235*N_235*(1-si_f235*phi*x)/(si_a235*N_235*(1-si_a235*phi*x)+si_a238*N_238*(1-si_a238*phi*x))==n;
t = vpasolve(eqn , x)
t =
47756908.067411697270732667471209
##### 3 CommentsShow 1 older commentHide 1 older comment
SETH SCHREINER on 28 Feb 2023
oh i dont have the right toolbox apparently
Anton Kogios on 28 Feb 2023
Yes sorry, this requires the Symbolic Math Toolbox, which is very helpful for these sorts of problems. You could of course always follow Torsten's advice in his answer.

Torsten on 28 Feb 2023
Edited: Torsten on 28 Feb 2023
You don't need a toolbox for this.
Take a piece of paper and a pencil, multiply the equation by the denominator of the left-hand side, collect terms with x on one side and other terms on the other side, factor out x on the one side and solve for x.
It's just like solving 3*x + 5 = 7*x - 4 for x.
SETH SCHREINER on 28 Feb 2023
I get how to do it on pencil and paper, but the equation gets rather tedious trying to separate out x from the other variables towards the end

### Categories

Find more on Linear Algebra in Help Center and File Exchange

R2022b

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!