Maths Olympiad Prep

Library / /75 of 133

Algebra Difficulty 5.7 AIME, harder Prove it Saudi Arabia

1n=0.a1a2 \frac{1}{n}=0.a_{1}a_{2}\ldots
Suppose that n=a1+a2+n=a_{1}+a_{2}+\cdots. Determine all possible values of nn.

Solution

Let n2n \geq 2 be such an integer. Because n=a1+a2+n=a_{1}+a_{2}+\cdots, there exists a positive integer k>0k>0 such that ak0a_{k} \neq 0 and ai=0a_{i}=0 for all integers i>ki>k.

Because 10kn=a1a2ak\frac{10^{k}}{n}=\overline{a_{1}a_{2}\cdots a_{k}} is an integer and 10k1n=a1a2ak1,ak\frac{10^{k-1}}{n}=\overline{a_{1}a_{2}\cdots a_{k-1},a_{k}} is not an integer, there exist two non-negative integers a,b0a, b \geq 0 such that n=2a5bn=2^{a} \cdot 5^{b}, with max{a,b}=k\max\{a, b\}=k. We have three cases:

a. If k=a=bk=a=b, then n=10k10n=10^{k} \geq 10 and a1+a2++ak=1na_{1}+a_{2}+\cdots+a_{k}=1 \neq n, contradiction.

b. If k=b>ak=b>a, then
5k2a5k=n=a1+a2++ak9k. 5^{k} \leq 2^{a} \cdot 5^{k}=n=a_{1}+a_{2}+\cdots+a_{k} \leq 9k.
For k=2k=2, we have 52=25>18=9×25^{2}=25>18=9 \times 2. If k2k \geq 2 and 5k>9k5^{k}>9k, then 5k+1>5×9k=9(k+4k)>9(k+1)5^{k+1}>5 \times 9k=9(k+4k)>9(k+1). This implies that k=1k=1 and therefore a=0a=0, that is n=5n=5. But 1n=0.2\frac{1}{n}=0.2 and n2n \neq 2, contradiction.

c. If k=a>bk=a>b, then
2k2k5b=n=a1+a2++ak9k. 2^{k} \leq 2^{k} \cdot 5^{b}=n=a_{1}+a_{2}+\cdots+a_{k} \leq 9k.
For k=6k=6, we have 26=64>54=9×62^{6}=64>54=9 \times 6. If k6k \geq 6 and 2k>9k2^{k}>9k, then 2k+1=2k+2k>9k+26>9(k+1)2^{k+1}=2^{k}+2^{k}>9k+2^{6}>9(k+1). This implies that k5k \leq 5.

Now, assume that b1b \geq 1. We have
10×2k1n=a1+a2++ak9k. 10 \times 2^{k-1} \leq n=a_{1}+a_{2}+\cdots+a_{k} \leq 9k.
For k=1k=1, this is not satisfied. Assume that 10×2k1>9k10 \times 2^{k-1}>9k. We have 10×2k>9k+10×2k19k+10>9(k+1)10 \times 2^{k}>9k+10 \times 2^{k-1} \geq 9k+10>9(k+1), contradiction.

Hence, b=0b=0 and n=2kn=2^{k}.

By checking for k=1,2,3,4,5k=1,2,3,4,5 we find that only n=23n=2^{3} has this property.

Hence n=8n=8.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.