# 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

• Have a look at Reduce. – b.gates.you.know.what 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

I think you are looking for Replace:
p4 - p5 == r45 i45 /. r45 -> 1