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Symbolic matlab: how to set a variable as a function of time?

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Sebastian D'Amico
Sebastian D'Amico 15 Feb 2012
Hello to all! I'm trying to set a symbolic variable as a function of "t". This is because I need to obtain something like this:
K>> diff(sin(theta),t)
ans = diff(theta,t)*cos(theta)
or
ans= theta_dot*cos(theta)
but how to tell Matlab that variable theta is a function of time "t"?
Thank you!

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Ezequiel Garcia
Ezequiel Garcia 18 Apr 2017
You need to declare t as a "symbolic"
syms t;

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답변(4개)

Walter Roberson
Walter Roberson 15 Feb 2012
syms theta t
diff(sin(theta(t)),t)

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Walter Roberson
Walter Roberson 19 Apr 2017
In newer versions of MATLAB, you can use
syms theta(t)
diff(sin(theta),t)
Sergei Sergienko
Sergei Sergienko 13 Jun 2020
Is it possible to make vector function of t? Something like this
sym('q(t)', [3 1])
diff(sin(q),t)

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Wayne King
Wayne King 15 Feb 2012
Hi Sebastian, do you know the expression for theta(t)?
syms theta t;
theta(t) = t^2;
diff(cos(theta),t)

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Sebastian D'Amico
Sebastian D'Amico 15 Feb 2012
Wayne, thank you for your answer! I don't have the expression of theta because I don't need it. This is the reason I need to tell Matlab that theta is a function of "t" so, when derivate is computed, it should consider that, for example, diff(sin(theta),t) is equal to diff(theta,t)*cos(theta). But, how to set it?
Seabstian

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Wayne King
Wayne King 15 Feb 2012
Do you really need that if you do not know the expression for theta(t), you know df(theta)/dt = df/dtheta*dtheta/dt

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sharara rehimi
sharara rehimi 16 Aug 2018
thanks a lot..great for my problem

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