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How can I solve the following derative problem in Matlab ?

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Vuqar Samedov
Vuqar Samedov 2020-10-4
评论： Bjorn Gustavsson 2020-10-6
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Vuqar Samedov
Vuqar Samedov 2020-10-4
编辑：Vuqar Samedov 2020-10-4
Dear Steven Lord,
I can not write the command correctly in the matlab program. I need help.
syms x t
>> y=(x^3+5*x-4);
>> x=(t^2+t);
>> t=-1;
>> dy_dt = diff(y,t)
Error using sym/diff (line 70)
Second argument must be a variable or a nonnegative integer specifying the number of differentiations.
Vuqar Samedov
Vuqar Samedov 2020-10-5
I am a student. I'm learning a new Matlab program. Friends, please help me solve this problem.

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Bjorn Gustavsson
Bjorn Gustavsson 2020-10-4
When you assign:
t = -1;
t becomes a standard double scalar, and is no longer a symbolic variable. So that's where you go completely wrong. Before that, when you describe your variable/function y it becomes a function of x, after that you assign x to some polynomial in t. That second assignment does not have "time-traveling" powers, it does not retroactively affect the definition for y, think of an assignment as "from here on forward...". Try to change order of your definitions...
HTH

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Vuqar Samedov
Vuqar Samedov 2020-10-6
  2 个评论
Bjorn Gustavsson
Bjorn Gustavsson 2020-10-6
You are pretty much on the right track.
1, Define your symbolic variables.
2, Look at the problem at hand, not that y depends on x that in turn, depends on t.
3, try do the assignments to y and x in different order, and see what happens when you run diff(y,x) and diff(y,t).
4, if something get you closer to the solution, have a look at how you can proceed from there.
5, a useful function might be matlabFunction - have a look at the help and documentation for that one.

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