# Evaluating symbolic expression, when some variables are finally fixed

Hello

I'm looking for solution when bigger number of variables is in the picture.

Simple example:
There is relationship I = V/R
then we know that R = 3, with that knowledge let's plot I(V)

in sage it should look something like:

```
V = var('V')
R = var('R')
I = V/R
R = 3
plot(I)
```

but above is not working, because in plot(I) I is still V/R not V/3 (knowledge of R=3 is not used), so I'm looking for some operation that turns V/R into V/3, something like

```
I = evaluate(I)
```

but such evaluate() apparently does not exist

there are however two different ways

(1) repeat I=V/R after R=3 and in such situation I becomes V/3 as needed

(2) I = I(R=R) and with earlier R=3, I also becomes V/3

both however seems for me be solution(s) for simple expression with just few variables, and not good (not as straightforward as hypothetical I = evaluate(I) for more complex expression with several variables.

Is such evaluate() or so, available already, and I failed to find it?

Is there a (easy) way to implement that proposed evaluate() ?