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awarded  Notable Question
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Nov
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accepted Is the empty set the only possible set for $A$ such that $A=\{x|x\not\in A\}$?
Nov
27
comment Is the empty set the only possible set for $A$ such that $A=\{x|x\not\in A\}$?
The most understandable answer. :-) Thanks.
Nov
27
asked Is the empty set the only possible set for $A$ such that $A=\{x|x\not\in A\}$?
Oct
20
answered arrange m balls in to n baskets
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awarded  Explainer
Sep
23
reviewed Approve Which was defined first to represent $\underbrace{a+a+a+\cdots+a+a+a}_{n \text{ terms}}$? $n\times a$ or $a \times n$?
Sep
23
asked Which was defined first to represent $\underbrace{a+a+a+\cdots+a+a+a}_{n \text{ terms}}$? $n\times a$ or $a \times n$?
Sep
11
awarded  Popular Question
Aug
27
asked Are there other methods to evaluate $\frac{1^{-4}+2^{-4}+3^{-4}+4^{-4}+\cdots}{1^{-4}+3^{-4}+5^{-4}+7^{-4}+\cdots}$?
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Aug
9
comment How to test whether a set of four points can form a parallelogram
We can save the calculation just by calculating the midpoint times 2 to avoid dividing by 2.
Aug
9
comment How to test whether a set of four points can form a parallelogram
How about the case there are 3 points on a line?
Aug
9
accepted How to test whether a set of four points can form a parallelogram
Aug
9
comment How to test whether a set of four points can form a parallelogram
But all of your three tests may be false even though the given 4 points can be vertices of a parallelogram.