# Who can explain why x= 1 = 0.99999 ....

• Nov 1st 2013, 11:29 AM
SelfTaughtMaths
Who can explain why x= 1 = 0.99999 ....
Lets say

10x = 9.99999999 ... , then x= 0.999999999 ....

Thus, 9x = 10x - x = 9.9999999... - 0.9999999.... = 9
thus, since 9x = 9 , then x=1

Wait a second, I thought x=0.99999999 ....

0.999999... does not equal 1
• Nov 1st 2013, 11:54 AM
SlipEternal
Re: Who can explain why x= 1 = 0.99999 ....
Decimal representations are not unique. Every number that terminates with an infinite string of zeros can be represented by an expression that terminates with an infinite string of 9's. To understand this further, you would need to learn some topology and/or analysis. The equality comes from something called the $\varepsilon$-principle. Given two numbers $x,y$, if for all $\varepsilon>0$, $|x-y|<\varepsilon$, then $x=y$. Here, $|x-y|$ is the absolute value function, so I am referring to real numbers. If you are using a different metric $d$, then if $d(x,y)<\varepsilon$ for all $\varepsilon>0$, you have $x=y$.
• Nov 1st 2013, 12:02 PM
HallsofIvy
Re: Who can explain why x= 1 = 0.99999 ....
Yes, 0.999999... is equal to 1. And 0.5999... is equal to 0.6 etc.
• Nov 2nd 2013, 01:21 AM
topsquark
Re: Who can explain why x= 1 = 0.99999 ....
Quote:

Originally Posted by SelfTaughtMaths
Lets say

10x = 9.99999999 ... , then x= 0.999999999 ....

Thus, 9x = 10x - x = 9.9999999... - 0.9999999.... = 9
thus, since 9x = 9 , then x=1

Wait a second, I thought x=0.99999999 ....

0.999999... does not equal 1

Try this as well:
$\frac{1}{3} = 0.33333333...$ if you do the division.

But then:
$3 \cdot \frac{1}{3} = \frac{3}{3} = 1$

and
$3 \cdot 0.333333333.... = 0.99999999,,,,,$

-Dan
• Nov 2nd 2013, 01:37 AM
Prove It
Re: Who can explain why x= 1 = 0.99999 ....
Quote:

Originally Posted by SelfTaughtMaths
Lets say

10x = 9.99999999 ... , then x= 0.999999999 ....

Thus, 9x = 10x - x = 9.9999999... - 0.9999999.... = 9
thus, since 9x = 9 , then x=1

Wait a second, I thought x=0.99999999 ....

0.999999... does not equal 1

You have to understand the basic rule of mathematics that if a = b and b = c, then a = c. In other words, if two things are equal to the same thing, then they are equal to each other.

Since you have shown that x is equal to both 0.9999... and 1, then that means 0.9999... = 1.
• Nov 2nd 2013, 08:12 AM