Why I got complex answer while I know it should be real????

Question

Fatemeh Eslami 2013-1-29

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/60259-why-i-got-complex-answer-while-i-know-it-should-be-real

Hi

I have a function to be solved. while I know it has a real answer I got complex answer, I check with 2 of my friends, one got real answer and the other one get the complex one as mine. what should I do? by the way,I want to solve it symbolic, This is the function and answer:

commands:

syms('x')
F=0.21*(exp(-0.157-0.157*x))-x
 s=solve(F,x)
 X=subs(s)

answer:

F = 21/(100*exp((157*x)/1000 + 157/1000)) - x
 s =(1000*lambertw(0, -(100000*exp(157/1000))/3297))/157
 X = 15.0552 +15.0159i

the answer tha my friend got with the same commands:

F =(21*exp(- (157*x)/1000 - 157/1000))/100 - x
 s =(1000*lambertw(0, (3297*exp(-157/1000))/100000))/157
 X = 0.1746

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

owr 2013-1-29

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/60259-why-i-got-complex-answer-while-i-know-it-should-be-real#answer_72788

My guess, and this may be a long shot, is that somewhere along the way you took the sqrt of -1.

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Fatemeh Eslami 2013-1-29

thanks, but no, I dont. In fact, I have just write that one line command in my question.

请先登录，再进行评论。

Answer 2

Shashank Prasanna 2013-1-29

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/60259-why-i-got-complex-answer-while-i-know-it-should-be-real#answer_72789

在 MATLAB Online 中打开

I got the same as your friend. Try the following

syms x F s X real

3 个评论
显示 1更早的评论隐藏 1更早的评论

Fatemeh Eslami 2013-1-29

we both open the same mfile and run it in just separate computers.

Fatemeh Eslami 2013-1-29

Thanks Benji Bowbow,

this works:) syms x F s X real just I want to know that if in other cases that I am not sure about the complexity or reality of the answers,using this method gives me the correct answer or not.

请先登录，再进行评论。

Answer 3

Walter Roberson 2013-1-29

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/60259-why-i-got-complex-answer-while-i-know-it-should-be-real#answer_72794

Both F are the same, but your solve() is returning the wrong answer. The lambertw argument is of the form A*exp(-B) but your solve() is instead forming 1/A * exp(B) .

Which MATLAB versions are the two of you running? Is one of you running with the Maple based symbolic toolbox, and the other running with the MuPAD based symbolic toolbox?

Note: complex values are not inherently wrong solutions. The general solution is not restricted to LambertW branch 0 the way the two solutions here show.