# Solve an equation and get a solution that depends on a variable

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Jonas Ström Seez 2020 年 3 月 28 日
コメント済み: Jonas Ström Seez 2020 年 3 月 29 日
Hi everyone! I need some help with getting a solution to a equation. I am trying to solve a equation using solve. However i dont get any solution to the equation, i only get Empty sym: 0-by-1. I can solve the equation manually, the problem is that i have other equation that i cant solve manually and would like a better way to solve them. Is there any good way to solve a equation and get a solution that has a dependent variable in return. Here is what i have tried so far.
syms Nin(t)
% Vin = (sign(t - 200/11)/2 + 1/2)*((11*t)/100000 - 1/500) - (11*t)/100000 + 1/500
eq1 = Nin == Constant * Vin;
sol = solve(eq1, t);
sol -> Empty sym: 0-by-1
Manually
Nin = Constant * Vin;
-> Nin = Constant * (sign(t - 200/11)/2 + 1/2)*((11*t)/100000 - 1/500) - (11*t)/100000 + 1/500
Any guidance is appreciated.
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Torsten 2020 年 3 月 28 日

syms Nin t
syms Nin(t)
Jonas Ström Seez 2020 年 3 月 29 日
It worked! Thank you very much!

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### 回答 (1 件)

Walter Roberson 2020 年 3 月 28 日
syms Nin(t)
% Vin = (sign(t - 200/11)/2 + 1/2)*((11*t)/100000 - 1/500) - (11*t)/100000 + 1/500
eq1 = Nin == Constant * Vin;
sol = solve(eq1, t);
That code requests that an unknown function, Nin(t) be compared to Constant times an expression in t, and that the result be solved for the t that makes the two sides equal. The result would implicitly be of the form
t = some function of Nin(t) and Constant * Vin
except it cannot be, because the right hand side of solving for t cannot contain t, but it must because Nin(t) is an unknown function of t.
Therefore solve() must fail.
Nin = Constant * Vin;
-> Nin = Constant * (sign(t - 200/11)/2 + 1/2)*((11*t)/100000 - 1/500) - (11*t)/100000 + 1/500
That is not a solution for t, it would simply make the two sides of eqn1 the same as each other, in which case solving for t would have to give all t.

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