Can mathematica simplify an expression setting simplified answer equal to zero?

Question

Basically I want to reduce something like:

Subscript[P, 4] - Subscript[P, 5] == Subscript[R, 4, 5]* Subscript[i, 4, 5]

when only one of them is known (Subscript[R, 4, 5] == 1), so that Mathematica expresses the simplified form as:

Subscript[i, 4, 5] + Subscript[P, 5] - Subscript[P, 4] == 0

Yeah I tried that... But how can you tell it to reduce so you have constants on one side ? — TehHO, Oct 25 '12 at 09:30
Perhaps this is what you are looking for? Is is possible to have mathematica move all terms to one side of an equation? — Michael Wijaya, Oct 25 '12 at 09:34
Yeah, but I also have some expressions which equal 1, and I do not want that moved over setting everything to equal zero...
Basically I want all unknown on one side. And all that's known (defined) on the other side.. — TehHO, Oct 25 '12 at 09:39
So you have a - b == c * d and your only known variable is c. Then, according to your last comment, the output should be: (a - b)/d == c. That is what you are looking for? — István Zachar, Oct 25 '12 at 09:48
i have : a - b == c * d
c = 1,

therefore, by simplifying mathematica outputs:

a = d + b

Can i output that as

0 = (d+b)/a ?

All known on one side, all unkown on the other side... — TehHO, Oct 25 '12 at 09:58
using Michael's link Head[#][Subtract @@ #, 0] &@(a - b == c*d) — Dr. belisarius, Oct 25 '12 at 12:01
But still using that method, when I define a, I want it to fly to the left hand side... as I now know this value it's no longer unknown? Does that make sence?
The thing is.. have all these equations ´where a-b==c*d´ and I'm trying to build a matrix, therefore i need to have knowns on one side, and unknowns on the other side.

see this: http://i.imgur.com/jgzo7.png — TehHO, Oct 25 '12 at 15:27

score 1 · Answer 1 · answered Mar 03 '15 at 11:16

1

I think you are looking for Replace:

p4 - p5 == r45 i45 /. r45 -> 1
(* p4 - p5 == i45 *)

% /. lhs_ == rhs__ -> rhs - lhs == 0
(* i45 + p5 - p4 == 0 *)

answered Mar 03 '15 at 11:16

djp

0 = (d+b)/a ?