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(本小题满分13分) 设数列的各项为正数，前项和为，且. (Ⅰ)求数列的

(本小题满分13分)

设数列的各项为正数，前项和为，且.

(Ⅰ)求数列的通项公式；

(Ⅱ)记，若，求正整数的最小值.

答案

解：(Ⅰ)由，平方得，∴.

两式相减得，整理得，

即.∵，∴，即.

又∵当时，，，∴，

∴. …………………………7分

(Ⅱ) ∵.　…………………………10分

∴

∴，解得，

∴正整数的最小值为5. …………………………13分

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(本小题满分13分) 设数列的各项为正数，前项和为，且. (Ⅰ)求数列的

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