Re: [微積] [1,∞) 是開還是閉？
※ 引述《ntme (ntme)》之銘言：
: 想請問[1,∞)是開集合還是閉集合，or非開非閉
: (-∞,∞)是開集合嗎？
: 一直搞不清楚∞的點...
Let |R = (-∞,∞).
[1,∞) is a closed set in |R since its complement (-∞,1) relative to |R
is an open set in |R.
|R = (-∞,∞) is both an open and a closed set. Think about its complement
relative to |R, leave you as an exercise.
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