Derivate an expression with time

Good Morning,

My problem is I have an equation depends on theta and I don't have an expression of theta (which is the variable in the equation), and theta depends on t and i like derivate my equation with t.

For example my equation is as follows : f (t) = a * cos (theta)

Where theta = theta (t) And I would like to have a result in this style : f '(t) = - a * (theta)' * sin(theta)

Thank you.

Answer 1

[Walter Roberson]

syms a theta(t) theta_prime f_prime
f(t) = a * cos(theta);
dtheta = diff(theta,t);
df = diff(f,t);
f_prime == subs(df, dtheta, theta_prime)

[Anis Frej]

Think you verry much ;)