solve not returning two answers
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Hi,
I have a nonlinear equation that I need to sovle for 'x'. Following is my code:
syms x q N
q=.95;
N=500;
eqn = 2*log((((N-x)/(q*N))^(N-x))*((x/((1-q)*N))^x)) == 3.841;
solx = solve(eqn, x);
I am expecting two values for 'x' but am getting just one (i.e. ~35). I am not getting the other value of 16. Also I get the following warning:
Warning: Cannot solve symbolically. Returning a numeric approximation instead.
Please tell me what I am doing wrong.
Thanks,
0 件のコメント
採用された回答
infinity
2019 年 7 月 7 日
Hello,
Maybe you could try with another approach, for example,
eq = @(y)2*log((((N-y)./(q*N)).^(N-y)).*((y./((1-q)*N)).^y)) - 3.841;
fsolve(eq,[0 100])
It will give two solutions.
ans =
16.0510 35.1056
But in this case, you should be aware that if you chose different interval of solutions, for example [-100, 100] in fsolve, you may get wrong results.
その他の回答 (1 件)
Walter Roberson
2019 年 7 月 8 日
When an equation is sufficiently complicated, solve cannot find all the exact solutions. Often it does not even know how to find one exact solution. In such cases, solve() uses vpasolve() to find one numeric solution.
2 件のコメント
Walter Roberson
2019 年 7 月 8 日
It is always a mistake to use solve() for equations with floating point coefficients such as 3.841. solve() is for finding exact solutions, which is something that you cannot do for equations involving inexact quantities such as 3.841. 3.841 represents a range of numbers, everything from 38405/10000 to 38149/10000. 3.841 does *not* mean 3841/1000 exactly.
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