$\"\"$

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Draw a diagram for situation:

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$\"\"$

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Apply the Cosine Law: $\"\"$ .

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

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ .

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The footing should be approximately is $\"\"$ apart.

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$\"\"$

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The footing should be approximately is $\"\"$ apart.