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Algebra Difficulty 6.5 National Olympiad Prove it Belarus

Given a decimal fraction dd such that there are only digits 0, 1, and 2 in its decimal representation. It is known that if each 0 in the decimal representation of dd are replaced by 1, then the obtained decimal fraction is periodic; if each 1 in the decimal representation of dd are replaced by 2, then the obtained decimal fraction is periodic too.
Can one claim that the decimal fraction dd is periodic ?
(M. Karpuk)

Solution

Answer: yes, one can.
Suppose that if we replace each 0 in the decimal fraction dd by 1 we obtain the decimal fraction aa, and if we replace each 1 in the decimal fraction dd by 2 we obtain the decimal fraction bb. Since both the fractions aa and bb are periodic, we see that both these numbers are rational numbers.
If we replace each 2 in the decimal fraction aa by 0 we obtain some decimal fraction α\alpha which is evident periodic and so α\alpha is a rational number. Similarly, if we replace each 0 in the decimal fraction bb by 1 we obtain some decimal fraction β\beta which is also periodic, and so β\beta is a rational number too.
It is easy to see that d=βαd = \beta - \alpha. Therefore, dd is a rational number and so the decimal fraction dd is periodic.

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