Step 1:

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Draw the related diagram:

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$\"\"$

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The distance between points $\"\"$ and $\"\"$ is $\"\"$ .

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The angles $\"\"$ and $\"\"$ .

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The sum of angles in a triangle is $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ .

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Find the distance across river $\"\"$ .

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Apply law of sines: $\"\"$ .

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Consider $\"\"$ .

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Substitute

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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 distance across river is $\"\"$ .