# Equations solving This post is a wiki. Anyone with karma >750 is welcome to improve it.

Hi everybody, i've got this two equations:

eqq0: V_f*cos(delta(t))=v(t)
eqq1: V_f*sin(delta(t))=l*diff(psi(t))


I need to isolate V_f from the first equation, obtaining V_f=v(t)/cos(delta(t)) and substitute it in the second equation. Then i have to isolate diff(psi(t))=[v(t)*tan(delta(t))]/l.

I did :

a=solve (eqq0,V_f)
V_ff=a.right()


but it says "AttributeError: 'Sequence_generic' object has no attribute 'right'"

How can i solve this two equations?

Thank you.

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ciao Silvia,

use, e.g.,

a.right()


to get the RHS of the first solution. Indeed, solve returns a list of solutions.

more

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?