Equations solving

edit

It works! Grazie mille! ;)

Now i substituted that value in the second equation doing eqq1.left().subs(V_f==V_ff)

but when i do

solve(eqq1,diff(psi))

it gives me diff(psi(t))=V_f*sin(delta(t))/l

like i never did that substitution before.

What can i do to have diff(psi(t))=[v(t)*tan(delta(t))]/l as a solution?

Be sure to redefine eqq1 with the substituted expression. That is, eqq1=eqq1.left().subs(V_f==V_ff).

I redefined eqq1, but doing like this it ignores the left part of the equation, so eqq1 becomes just sin(delta(t))*v/cos(delta(t))

Ok i got it, i just had to writeeqq1=eqq1.subs(V_f==V_ff)

Now the problem is that when i do solve(eqq1,diff(psi(t))) he doesn't solve it (it gives me [ ]), and if i do solve(eqq1,diff(psi)) is says No differentiation variable specified.

What's wrong?