$\"\"$

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The function is $\"\"$ .

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$\"\"$

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$\"\"$

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Apply Summation formula $\"\"$ .

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$\"\"$

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Cancel common terms.

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$\"\"$

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$\"\"$

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$\"\"$

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As $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ .