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## Homework Statement

prove (a,b) x (c,d) is open in R^2

## Homework Equations

## The Attempt at a Solution

given an x that belongs to (a,b)x(c,d) there exists a delta such that given any y that belongs to R^2 whose d2(x,y)< epsilon, y also belongs to (a,b)x(c,d). Need to prove this.