$\"\"$

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From diagram : $\"\"$ and $\"\"$ .

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Now find angle $\"\"$ .

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Apply cosines law : $\"\"$ .

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

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$\"\"$

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Divide each side by $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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Now find angle $\"\"$ .

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Apply cosines law: $\"\"$ .

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

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$\"\"$

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Divide each side by $\"\"$ .

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$\"\"$

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$\"\"$

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Now find angle $\"\"$ .

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In triangle $\"\"$ , $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$ .