solve equation for a variable

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SETH SCHREINER
SETH SCHREINER 2023-2-28
评论： Anton Kogios 2023-2-28
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

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采纳的回答

Anton Kogios
Anton Kogios 2023-2-28
Ran in:
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
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SETH SCHREINER
SETH SCHREINER 2023-2-28
oh i dont have the right toolbox apparently
Anton Kogios
Anton Kogios 2023-2-28
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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更多回答（1 个）

Torsten
Torsten 2023-2-28
编辑：Torsten 2023-2-28
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 个评论
SETH SCHREINER
SETH SCHREINER 2023-2-28
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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