Hello, I have to do a substitution in an expression: i want to substitute diff(psi) with omega, where i declared psi as psi = function('psi',t)
I tried doing expr.substitute(diff(psi)==omega) but it doesn't work.
What do I have to do?
Thank you very much.
[Edited to answer your second question as well.]
To replace diff(psi) by say t, use substitute_expression as follows:
sage: t = var('t')
sage: psi = function('psi',t)
sage: a = 3 * diff(psi)^2 - 2 * diff(psi) + 1
sage: a
3*D[0](psi)(t)^2 - 2*D[0](psi)(t) + 1
sage: a.substitute_expression(diff(psi)==t)
3*t^2 - 2*t + 1To use the result of solve to perform replacement, do as follows:
sage: var('V V_f chi l psi')
sage: eq0 = V == V_f * cos(chi)
sage: eq1 = l * psi == V_f * sin(chi)
sage: sol = solve(eq0, V_f)
sage: sol
[V_f == V/cos(chi)]
sage: eq0.subs_expr(sol[0])
V == V
sage: eq1.subs_expr(sol[0])
l*psi == V*sin(chi)/cos(chi)Note that sol is a list containing one equation. We use that equation (item 0 of the list) for the replacement.
Thank you!
Can i ask you another question? I've got two equations:
eq0=V==V_f*cos(chi)eq1=l*psi==V_f*sin(chi)
and i did x=solve(eq0, V_f), so x=v/cos(chi).
Now i want to substitute the value of V_f that i've just calculated into the eq2.
How can i do that?
The x is a list, so the content you want is got with x[0]. This is an equation, and you want the right hand side of that, so you say x[0].rhs().
Asked: 10 years ago
Seen: 17,101 times
Last updated: Dec 06 '14
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