$\"\"$

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observe the diagram :

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The sides of the triangle are $\"\"$ , $\"\"$ and $\"\"$ .

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Find the angle $\"\"$ .

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Apply cosines law : $\"\"$ .

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

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$\"\"$

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$\"\"$ .

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$\"\"$

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Find the angle $\"\"$ .

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Apply cosines law : $\"\"$ .

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

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$\"\"$

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$\"\"$ .

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$\"\"$

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Find the angle $\"\"$ .

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In $\"\"$ the sum of angles is $\"\"$ .

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

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$\"\"$

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$\"\"$ .

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$\"\"$

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