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
Rewriting, you have
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
Rewriting, you have
and this is equivalent to