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## Main Question or Discussion Point

I think cos(x) is closed function in R.

But I heard that cos(x) is not closed function in R.

What do I choose closed set A in R, cos(A) is not closed in R?

Help....

But I heard that cos(x) is not closed function in R.

What do I choose closed set A in R, cos(A) is not closed in R?

Help....