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i'm trying to solve system of 9 coupled linear differential equations.I've this problem ... what is it's meaning? enter image description here

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    $\begingroup$ Go ahead and post the code that leads to the error. $\endgroup$
    – Jason B.
    Sep 8, 2015 at 11:25
  • $\begingroup$ Greetings! Make the most of Mma.SE and take the tour. Help us to help you, write an excellent question. Edit if improvable, show due diligence, give brief context, include minimum working examples of code and data in formatted form. As you receive give back, vote and answer questions, keep the site useful, be kind, correct mistakes and share what you have learned. $\endgroup$
    – rhermans
    Sep 8, 2015 at 11:34
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    $\begingroup$ Potential duplicates: Set::write posts -- Sorry no time to sift them....Ahmed, have you seen them? $\endgroup$
    – Michael E2
    Sep 8, 2015 at 12:47
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    $\begingroup$ You probably used a = somewhere where a == would have been appropriate. $\endgroup$ Sep 8, 2015 at 14:44
  • $\begingroup$ More potential duplicates: error deqn posts; in particular, (40314). Hard to tell, with the question in its current incomplete state. Have you searched for these things? Do any solve your problem? $\endgroup$
    – Michael E2
    Sep 8, 2015 at 14:52

1 Answer 1

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You are likely using = (Set) instead of == (Equal) in the equations you are feeding to DSolve. You need to use ==, which is an equality test. Instead, you're trying to assign the right-hand side (presumably zero) to the linear combinations of As reported by Set::write. These linear combinations have the overall head Plus, which means that you're ultimately telling Mathematica something like

Set[Plus[...],0]

Here Plus is Protected, which means that Set can't assign it to anything - hence the error as reported.

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