1. The problem statement, all variables and given/known data prove (a,b) x (c,d) is open in R^2 2. Relevant equations 3. 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.