# solve equation for a variable

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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

##### 0 Comments

### Accepted Answer

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)

##### 3 Comments

Anton Kogios
on 28 Feb 2023

### More Answers (1)

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.

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