(3)、记 ,求数列的前项和,并求                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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