I was doing some topology problems and well they were asking me to find the closure, interior and so on. The problem is not really asking for a justification on my part, but what if it did?
For example given $A=\{(x,y)\; | \; x^2+y^2<1\}$
if it asked me to find the closure I would just write this: $\bar{A}=\{(x,y)\; | \; x^2+y^2 \leq 1\}$
but what else could I add to justify the above.