2. Let
denote the sum of divisors of natural number
, including
and
. For every
define
as number of natural numbers
, for which
is odd number. Prove that there are infinitely many natural numbers
, such that
.
3. Let b) Let
and
be feet of perpendiculars from
on the lines
and
and
foot of perpendicular from
on
. Let
be intersection point of
and
. Prove that
.
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