I have to prove that the arbitrary union of open sets (in R) is open.(adsbygoogle = window.adsbygoogle || []).push({});

So this is what I have so far:

Let [itex]\{A_{i\in I}\}[/itex] be a collection of open sets in [itex]\mathbb{R}[/itex]. I want to show that [itex]\bigcup_{i\in I}A_{i}[/itex] is also open...

Any ideas from here?

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# Union of open sets question

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