wrong answer of solve function

2 次查看（过去 30 天）
iridium
iridium 2015-3-14
编辑： Star Strider 2015-3-14
When I use "solve" to find the root for the equation 1/x+(1/x)^3==1, it returns 3 answers, even including one that's negative. Yet obviously this equation should have unique solution. How should I refine the answer? Thank you!

请先登录，再回答此问题。

采纳的回答

Star Strider
Star Strider 2015-3-14
Why should it have a unique solution?
It’s a cubic equation, so it will have three roots. They do not have to be distinct roots, but there have to be three of them:
x =
1.4655712318767680266567312252199
- 0.23278561593838401332836561260997 - 0.79255199251544784832589830065336*i
- 0.23278561593838401332836561260997 + 0.79255199251544784832589830065336*i
  2 个评论
iridium
iridium 2015-3-14
Thank you for your comment! Sorry for not being clear, I was talking about real solutions. Since solve function returns three real answers, I thought it solves for reals.
Star Strider
Star Strider 2015-3-14
编辑：Star Strider 2015-3-14
My pleasure!
The solve function (I used vpasolve) doesn’t return three real answers, at least to your polynomial. It returns one real and two complex conjugate solutions.
It solves for all three solutions (as does the roots function if you cast your equation as a polynomial vector). Your equation does have a real solution: 1.46557123.
If you want only the real root, that’s easy enough:
p = [1 -1 0 -1];
pr = roots(p);
real_root = pr(imag(pr) == 0)
produces:
real_root =
1.4656

请先登录，再进行评论。

更多回答（0 个）

类别

在 Help Center 和 File Exchange 中查找有关 Linear Algebra 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by