$\"\"$

\

The trigonometric expression is $\"\"$ .

\

Consider $\"\"$ .

\

Replace $\"\"$ with $\"\"$ .

\

$\"\"$

\

The exact value of trigonometric function : $\"\"$ .

\

$\"\"$

\

$\"\"$

\

$\"\"$ .

\

$\"\"$

\

The solution is $\"\"$ .