For a set contained in the real numbersR, IfR -S is open, then S is closed.

I have no idea where to start on this!

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- Apr 3rd 2012, 07:16 PMKingdu8457Open/closed sets of Real Numbers
For a set contained in the real numbers

**R**, If**R -**S is open, then S is closed.

I have no idea where to start on this! - Apr 4th 2012, 07:13 AMSylvia104Re: Open/Closed sets of real numbers
Let be a real number. If use the fact that is open to show that is not a limit point of It will then follow that if is a limit point of then proving that contains all its limit points and so is closed.

- Apr 4th 2012, 04:40 PMModusPonensRe: Open/closed sets of Real Numbers
Another way is to note that R-(R-S)=S, that is S is the complement of an open set. It helps to think better, sometimes, to write X-A as A^c. You will see the DeMorgan laws quicker as well as other stuff.