$\"\"$

\

The sum is $\"\"$ .

\

Apply summation formula $\"\"$ .

\

$\"\"$ .

\

In this case $\"\"$ .

\

$\"\"$ .

\

$\"\"$

\

Graph:

\

$\"image\"$

\

Observe the graph:

\

The sum is

\

$\"\"$

\

$\"\"$ .