# Mathematica - (Very) Simple Question

• Mathematica
Hi!
I'm doing my first steps in Mathematica and I've wanted to do something simple like assigning a variable to the output and then use it. Something like this:

Code:
In:Vp = vprx == kprx2*X1;
In:dX1 = kp X0 + X1 (-cat kcatl - kgpx gpx - kprx2 prx2);
In:rX1 = Solve[dX1 == 0, X1];
Out:{{X1 -> (kp X0)/(catalase kcatl + gpx kgpx + kprx2 prx2)}}
then i want to Sbustitute X1 in the expression Vp, with the value of the output of rX1.I'm using the function Replace but this doesn't do what I want. The output is:
Code:
In:Replace[Vp, X1 -> rX1]
Out:vprx == kprx2 X1
when it should be
Code:
Out:vprx==(kprx2kp X0)/(catalase kcatl + gpx kgpx + kprx2 prx2)
Thanks a lot for all the help!

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There are two problems, one is simple, one is more complicated.

First, use "ReplaceAll" instead of "Replace", it uses a deeper algorithm that will properly do what you want it to do.

Second, look at the result of "rX1". It is NOT just a number that can be plugged in. It is a list that has one component with a variable, "X1", being transformed to a larger expression.

Try using:

Code:
ReplaceAll[Vp, rX1[] ]
This takes the first component of the rX1 list and uses it in the ReplaceAll function.

Try using:
Code:
ReplaceAll[Vp, rX1[] ]
This takes the first component of the rX1 list and uses it in the ReplaceAll function.
Awsome! It works just as I wanted! Thanks a lot!