||(x,y) - (a,b)|| is a norm measuring the distance from (a,b) to (x,y), what do you mean how is it defined? Distance between points in R^2, euclidean norm.
I am also interested in this question. I understand what is required for the proof, but I am just stuck as to how to manipulate:
|f(x,b) - f(a,b)|
into something that resembles δ??
We know that:
0 < ||(x,y) - (a,b) || < δ \Rightarrow |f(x,y) - f(a,b)| < ε
as the limit is equal to the...