Given a decimal fraction 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 are replaced by 1, then the obtained decimal fraction is periodic; if each 1 in the decimal representation of are replaced by 2, then the obtained decimal fraction is periodic too.
Can one claim that the decimal fraction is periodic ?
(M. Karpuk)
Solution
Answer: yes, one can.
Suppose that if we replace each 0 in the decimal fraction by 1 we obtain the decimal fraction , and if we replace each 1 in the decimal fraction by 2 we obtain the decimal fraction . Since both the fractions and are periodic, we see that both these numbers are rational numbers.
If we replace each 2 in the decimal fraction by 0 we obtain some decimal fraction which is evident periodic and so is a rational number. Similarly, if we replace each 0 in the decimal fraction by 1 we obtain some decimal fraction which is also periodic, and so is a rational number too.
It is easy to see that . Therefore, is a rational number and so the decimal fraction is periodic.
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