Hej, me and my study group have an issue regarding substituting time derivative of a variable with some more convinient expression.

Question

Jesper Ankersen 2020-10-1

评论： Ameer Hamza 2020-10-1

eqn = ............... + diff(x(t), t) + a*b*c...................;

So in the above equation there is a variabel named diff(x(t), t), this variable was introduced earlier in the script.

Now we want to substitute it with another more convinient name such as:

syms x_dot;

subs(eqn, 'diff(x(t), t)', x_dot);

But this gives a long list of errors, does anybody know why this is so difficult?

It is no problem to achieve when we substitute another variable from eqn such as:

syms x_dot;

subs(eqn, a, x_dot);

Answer 1

Ameer Hamza 2020-10-1

Don't input the symbolic expression as string. Following works

syms y(x) dy
eq1 = diff(y)^2
subs(eq1, diff(y, x), dy)

Jesper Ankersen 2020-10-1

Thanks alot sir, this fixed the problem

Ameer Hamza 2020-10-1

I am glad to be of help!