Tricky functions and how they function!

Algebra Level 3

0.999 = ? \large \lfloor 0.999 \dots \rfloor = ?

Note :

  • The ellipses mean that the 9 9 's go on forever

  • The context is in the real numbers

0 1 2 3 4 5 6 7

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3 solutions

Mohammad Farhat
Nov 5, 2018

Relevant wiki: Is 0.999... = 1?

A simple proof that 0.999 = 1 0.999 \dots = 1 is shown below:

PROOF

x = 0.999 x = 0.999 \dots then,

10 x = 9.999 10x = 9.999 \dots ,

If we subtract x x from both sides then,

9 x = 9 9x = 9

Divide both sides by 9 9

x = 1 = 0.999 x = 1 = 0.999 \dots

And thus 1 = 1 \lfloor 1 \rfloor = 1


You can read more about it in the wiki

This solution is true, but I still think that the answer should be 0, because 0.99999... tends to 1, but never actually reaches it. Even if a number is infinitesimally smaller, it still is smaller.

Parth Sankhe - 2 years, 7 months ago

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NO! Check the wiki

Mohammad Farhat - 2 years, 7 months ago

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I get that it is equal to 1, but I don't think that the floor function should be defined for such expressions

Parth Sankhe - 2 years, 7 months ago
Otto Bretscher
Nov 5, 2018

k = 0 0.9 ( 0.1 ) k = 0.9 1 1 0.1 = 1 \sum_{k=0}^{\infty}0.9(0.1)^k=0.9\frac{1}{1-0.1}=1 and the integer part is 1 \boxed{1} .

@Otto Bretscher , can you help me in handling the reports?

Mohammad Farhat - 2 years, 7 months ago

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It's late at night but I will get to it tomorrow!

Otto Bretscher - 2 years, 7 months ago

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Thank you! It is 12:07 for me!

Mohammad Farhat - 2 years, 7 months ago

Im writing this post because I'm a bit confused. Let's formulate the problem as below.

l i m n 9 k = 1 n ( 0.1 ) k lim_{n\rightarrow \infty} \lfloor 9\sum_{k=1}^{n}(0.1)^k\rfloor

I do not think the limit of this expression is equal to 1 1 (it looks more like 0 0 ). However, if we write as below, then the answer is 1 1

l i m n 9 k = 1 n ( 0.1 ) k \lfloor lim_{n\rightarrow \infty} 9\sum_{k=1}^{n}(0.1)^k\rfloor

I do not know which one is correct, so, any comment is welcome.

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