Was doing few exercises and came up with a question. How to proof (x+y)/(x+y) = 1 using another method? The only way to prove is by definiition that (x+y) = a, so (x+y) = a/a and since a number divided by itself is 1 then a/a = 1? I mean, i'd like to prove by a way like this: x/(x+y) + y(x+y) = ... = 1