Step 1 :

Find the exact value of $\"\"$ by using double-angle formula.

Double-angle formula : $\"image\"$ .

From the figure:

$\"\"$ .

Step 2 :

Pythagorean theorem : $\"\"$ .

$\"\"$

The trigonometric functions :

$\"\"$ and $\"\"$ .

$\"\"$

The exact value of $\"\"$ is $\"\"$ .

Solution :

The exact value of $\"\"$ is $\"\"$ .