3) Let D = {(x, y) = R² | 0 < x² + y² ≤ 4}. Give all boundary points of D. Is D open, closed or bounded? Justify all your answers. The definition of open set is in your Ebook in section 13.2. The related definitions of closed and bounded set are as follows: Closed: A set D is closed if it contains all of its boundary points. Bounded: A subset D of R" is bounded if it is contained in some open ball D₁ (0). For example the interval [-1,5) is neither open nor closed since it contains some but not all of its endpoints. It is bounded since it is contained in the open interval D6 (0) = (-6, 6).

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