Error using str2sym (line 83) Unable to convert string to symbolic expression: L 1 (C 103): BADCH: Invalid character(s).

Eq1 = ('n + 2*d -((e*6.24*n)/(n^2 +6.24*n + 6.24*5.68e-5))- 2*((e*6.24*5.68e-5)/(n^2 + 6.24*n + 6.24*5.68e-5))–((h*1.7e-3*n)/(n^2 + 1.7e-3*n + 1.7e-3*6.55e-8))– 2*((h*1.7e-3*6.55e-8-)/(n^2 + 1.7e-3*n + 1.7e-3*6.55e-8))- 5.3e-8/n=0');
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The correct version is:

Eq1 = 'n + 2*d - ((e*6.24*n)/(n^2 + 6.24*n + 6.24*5.68e-5)) - 2*((e*6.24*5.68e-5)/(n^2 + 6.24*n + 6.24*5.68e-5))-((h*1.7e-3*n)/(n^2 + 1.7e-3*n + 1.7e-3*6.55e-8)) - 2*((h*1.7e-3*6.55e-8)/(n^2 + 1.7e-3*n + 1.7e-3*6.55e-8)) - 5.3e-8/n=0';

There are other problems. I will let you solve them.

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Star Strider 2018-8-15

I have no recent experience with DAEs.

It will be best if you open a new Question describing your current problem. There are others here much more knowledgeable with respect to DAEs in MATLAB than I am.

Dursman Mchabe 2018-8-15

Actually, I have been doing that for some time now, DAEs are just challenging. See the links below:

https://www.mathworks.com/matlabcentral/answers/412270-this-dae-appears-to-be-of-index-greater-than-1?s_tid=mlc_ans_email_view#comment_600007

https://www.mathworks.com/matlabcentral/answers/414912-error-using-vertcat-dimensions-of-arrays-being-concatenated-are-not-consistent?s_tid=mlc_ans_email_view#answer_332830

I only had success with IVPs and BVPs. That is the reason I am also trying DSOLVE coupled with SOLVE. It will give me analytical solutions for all the variables.

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