Answer:
look at the picture I sent
1.01x10^4 decimal notation
2 answers:
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When 
, you have
Now assume this is true for
, i.e.
and under this hypothesis show that it's also true for
. You have
In other words, there exists
such that
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and this is equivalent to
modulo 9, as desired.
Now assume this is true for
and under this hypothesis show that it's also true for
In other words, there exists
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and this is equivalent to