calculus – Is it true that \$A=int(A)\$ or \$A=int(A) cup fr(A)\$ and true that \$A^C=ext(A)\$ or \$A=ext(A) cup fr(A)\$?

I’m trying to understang some topology of $$mathbb{R}^n$$ space.

I want to know if it’s true that $$A=int(A)$$ or $$A=int(A) cup fr(A)$$ and true that $$A^C=ext(A)$$ or $$A=ext(A) cup fr(A)$$

Further more, if $$A=int(A)cup fr(A)$$, then $$A^C=ext(A)$$ and if $$A=int(A)$$, then $$A^C=ext(A)cup fr(A)$$