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f1(x) = (x-y)/(x+y) if x + y is not 0 and otherwise f1(x,y) = 0

f2(x,y) = (xy)/(x^2 + y^2) if (x,y) is not (0,0) and otherwise f2(0,0) = 0

f3(x,y) = (x^3 - y^3)/(x^2 + y^2) if (x,y) is not (0,0), and otherwise f3(0,0) is 0

Which of these is continuous

A) none B) f1 only C) f2 only D) f3 only E) all three

I know the defination of continuity for a single variable is the lim as x-> a of f(x) = f(a)

So i assume for two variables it should be the lim as (x,y) -> (a,b) of f(x,y) = f(a,b)

But I am not sure how to figure this out can someone help out please