$\"\"$

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The formula is $\"\"$ .

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Consider $\"\"$ .

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Apply product-to-sum property : $\"\"$ .

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$\"\"$

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$\"\"$

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If $\"\"$ , $\"\"$ . (Since $\"\"$ )

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Thus, $\"\"$ .

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$\"\"$

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If $\"\"$ , then

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$\"\"$

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$\"\"$ (Since $\"\"$ )

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Thus, $\"\"$ .

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$\"\"$

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$\"\"$ .