Thank you, I did not notice that you can put another ball inside the ball to make the proof work.
Using this, I was able to make a series of inequalities using the triangle inequality, and managed to prove that y \in B_\delta(y) \subset B_\epsilon(x) \subset S.
Homework Statement
For S \subset Rn, prove that S° is open.
Homework Equations
S° are all interior points of S.
The Attempt at a Solution
My class has only learned how to use balls to solve these types of problems (no metric spaces). So I need to choose an ε > 0 so that Bε(x) \subset...