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

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

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Have a look at Reduce. – b.gatessucks Oct 25 '12 at 9:29
Yeah I tried that... But how can you tell it to reduce so you have constants on one side ? – TehHO Oct 25 '12 at 9:30
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 9:34
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 9:39
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 9:48

## 1 Answer

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 *)

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(probably the question should have been closed?) – djp Mar 3 '15 at 11:19