$\"\"$

\

Show that the $\"\"$ coefficient $\"\"$ is $\"\"$ if $\"\"$ .

\

$\"\"$

\

Consider $\"\"$ .

\

Substitue the function $\"\"$ .

\

$\"\"$

\

$\"\"$

\

Consider $\"\"$ .

\

Apply product-to-sum property : $\"\"$ .

\

$\"\"$

\

$\"\"$

\

If $\"\"$ , $\"\"$ . (Since $\"\"$ )

\

Thus, $\"\"$ .

\

$\"\"$

\

If $\"\"$ , then

\

$\"\"$

\

$\"\"$ (Since $\"\"$ )

\

Thus, $\"\"$ .

\

$\"\"$

\

Note : All the terms will become zero except for the term $\"\"$ , because when $\"\"$ , $\"\"$ .

\

$\"\"$

\

$\"\"$

\

The $\"\"$ coefficient $\"\"$ is $\"\"$ if $\"\"$ .