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solve equation for a variable

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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

Accepted Answer

Anton Kogios
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 Comments
SETH SCHREINER
SETH SCHREINER on 28 Feb 2023
oh i dont have the right toolbox apparently
Anton Kogios
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.

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More Answers (1)

Torsten
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.
  1 Comment
SETH SCHREINER
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

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