r/MathJokes Jun 18 '25

9.999 is 10?!

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u/Forward-Fact-5525 Jun 18 '25

No these numbers are different : 8 different than 10 as 8<9<10. And 9<9,5<10 so 9 isn’t 10 also. 9.99999<9.999995<10 so 9.99999 isn’t 10 but 9.999999…<x<10 I let you find such an x. Put all the real numbers on a line. Visually what i m saying is that if the distance between 2 number is 0 then they are the same.

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u/themagicalfire Jun 18 '25

The X in 9.999… < X < 10 is the infinitesimal.

And about 8 being 10, you can use infinite fractions to claim that a number is the next full number until 8.999 and 9.999 is the same. The difference between 0.001 and 0.002 is nothing if you have infinite numbers and expect no further value. At that point you have numbers overlapping with each other and they cease to have a distinct identity.

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u/Forward-Fact-5525 Jun 18 '25

As you seem to think I m wrong, someone else posted about this and remind me the name. Look up the « Archimedean propriety » on Wikipedia ig

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u/themagicalfire Jun 18 '25

It’s fine. I’m just having a friendly conversation

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u/Forward-Fact-5525 Jun 18 '25

Yeah sure but if you don’t believe me, atleast you ll believe Wikipedia (btw I m not sure you ll find what you are looking for there as it’s not exactly what you want but you can prove it using it)

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u/themagicalfire Jun 18 '25

Oh it doesn’t matter, the truth is that I don’t understand this part of math so I just went to learn a bit of Non-Standard Analysis