$\"\"$

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Consider the triangle $\"\"$ .

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The three vectors are $\"\"$ and $\"\"$ .

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Draw the related figure:

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$\"\"$

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From the figure :

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$\"\"$ .

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$\"\"$

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Consider $\"\"$ be the midpoint of $\"\"$ and $\"\"$ be the mid point of $\"\"$ .

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Draw the vectors $\"\"$ and $\"\"$ .

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$\"\"$

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From the figure :

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$\"\"$ .

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

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$\"\"$

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$\"\"$

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Substitute $\"\"$ .

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$\"\"$ .

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$\"\"$

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$\"\"$ .