# Substitute ratios

Suppose that I have the expression (x+y+z)/y. I want to substitute the rules Phix -> x/y, Phiz ->z/y.

Is there a way to do this?

• ((x + y + z)/y // Expand) /. {x/y -> φ, z/y -> θ} Change the rules accordingly. Sep 15 '15 at 13:57

(x + y + z)/y /. {x -> y*Phix, z -> y*Phiz} // Simplify