$\"\"$

\

Consider $\"\"$ .

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Half angle formula : $\"\"$ .

\

$\"\"$ .

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Law of cosines : $\"\"$ .

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$\"\"$

\

$\"\"$

\

$\"\"$

\

Substitute $\"\"$ in equation (1).

\

$\"\"$

\

Hence $\"\"$ .

\

$\"\"$

\

$\"\"$ .