Show that the subsets of the plane are open:(adsbygoogle = window.adsbygoogle || []).push({});

1.) A= {(x,y)|-1<x<1,-1<y<1}

2.) C= {(x,y)|2<x^{2}+ y^{2}<4}

I have no clue on how to start with this problem.

I have another question. What does this notation imply. f(x,y)=some function.

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# Show subsets of the plane are open

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