$\"\"$ \ \

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The expression is $\"\"$ . \ \

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Consider the sequence is $\"\"$ . \ \

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First term of the sequence $\"\"$ . \ \

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Second term of the sequence $\"\"$ . \ \

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Third term of the sequence $\"\"$ . \ \

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Then nth term of the sequence $\"\"$ . \ \

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The formula for sum of sequences $\"\"$ . \ \

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$\"\"$ . \ \

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$\"\"$ \ \

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$\"\"$ .