$\"\"$

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

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Consider $\"\"$ .

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$\"\"$

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

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$\"\"$

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Substitute $\"\"$ in $\"\"$ .

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$\"\"$

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Difference rule of sine angle : $\"\"$ .

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$\"\"$

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$\"\"$

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Apply the formula $\"\"$ and $\"\"$ .

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$\"\"$

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Consider $\"\"$ .

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$\"\"$ .

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Substitute $\"\"$ in equation (1).

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Hence $\"\"$ .

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$\"\"$

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$\"\"$ .