Answer:
The proof makes use of congruences as follows:
Explanation:
We can prove this result using congruences module 3. First of all we shall show that
for all
. By induction we have
. For
we have

- Suppose that the statement is true for
and let's prove that it is also true for
. In fact,

Then induction we proved that
for all
. Then

From here we conclude that the expression
is divisible by 3.