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Number theory Difficulty 6.2 National olympiad Prove it

Example 2 Proof: Regardless of how many 3s are added between the two 0s in the number 12008, the resulting number is always a multiple of 19.

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Solution

Let a0=12008,an=12033n 3’s08,n=1,2,a_{0}=12008, a_{n}=120 \underbrace{3 \cdots 3}_{n \text { 3's}} 08, n=1,2, \cdots.
First, since
a0=19×632,a_{0}=19 \times 632,

it follows that
19a0.19 \mid a_{0} .

Second, suppose 19an19 \mid a_{n}, then from
an+110an=228=19×1219an+1\begin{array}{c} a_{n+1}-10 a_{n}=228=19 \times 12 \\ 19 \mid a_{n+1} \end{array}

Therefore, for all integers nn, the number ana_{n} is a multiple of 19.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.