answered 10 years ago
You have to specify the derivation variable t:
sage: diff(psi,t) v(t)*D[0](delta)(t)/l + sin(delta(t))^2*v(t)*D[0](delta)(t)/(l*cos(delta(t))^2) + sin(delta(t))*D[0](v)(t)/(l*cos(delta(t)))