If x belongs to s, then 8-x belongs to s If s has two elements, then s=

Because the set s satisfies that if x belongs to s, then 8-x belongs to s, and S is a single element set
Then there must be x = 8-x
That is, 2x = 8, x = 4
The process is so simple. Don't understand me again!

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1. When the elements in the set s are all natural numbers, and satisfy the proposition "if x belongs to s, then 8-x belongs to s", Try to write all of s with 3 elements? My question Why are there three elements? If you take 1, then 8-1 = 7, then 8-7 or return to 1, how can there be three elements?
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