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(a) Show that A = {$(x,y) \in \mathbb R^2 : x + y > 5$} is open.

(b) Show that B = {$(x,y) \in \mathbb R^2 : x.y \ge 5$} is closed. I couldnt solve in my homework.How should I start this proof? Should I take any arbitary pair satisfying the condition?

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