0.999.../Decimal multiplication by 10

Multiplying an infinite decimal by 10 is just as simple as multiplying an finite decimal by 10: every digit shifts one space to the left.

Assumptions

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Theorem

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Statement

If A = 0.a1a2a3 then 10 × A = a1.a2a3a4

Proof

We apply the definition of an infinite decimal as a series:

 

Next we apply the fact that a scalar multiple of a series can be computed term-by-term:

 

Next we shift the series:

 

But a0 = 0 by assumption, so we can simplify:

 

which is the desired result.