Friday, June 9, 2017

rational numbers - How can I explain $0.999ldots=1$?











I have to explain $0.999\ldots=1$ to people who don't know limit.




How can I explain $0.999\ldots=1$?



The common procedure is as follows



\begin{align}
x&=0.999\ldots\\
10x&=9.999\ldots
\end{align}




$9x=9$ so $x=1$.


Answer



What I always find the most simple explanation is:
$$
\frac{1}{3} = 0.333\ldots \quad \Longrightarrow \quad 1 = 3 \cdot \frac{1}{3} = 3 \cdot 0.333\ldots = 0.999\ldots
$$


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