$\"\"$

\

Statement: $\"\"$

\

Consider $\"\"$ .

\

Use the formula of law of cosines $\"\"$ and $\"\"$ .

\

Substitute $\"\"$ and $\"\"$

\

$\"\"$

\

$\"\"$

\

$\"\"$

\

$\"\"$

\

Therefore, $\"\"$ .

\

$\"\"$

\

$\"\"$ .