Question

show that:
A is open in X if and only if....A ∩ bd(A)=empty set

Answer 1

If A is open the boundaries of A are not included in the set A

Answer 2

so the intersection of A and the boundaries have nothing in common

Answer 3

therefore it is empty

Answer 4

but how doe the proof goes

Answer 5

I know that is true...but i have to prove it