⑵ The sum of the items in the sequence {bn} is the limit of the sum of the first n items, which can be expressed by the formula S=a 1/( 1-q). At this time, (1-q n) approaches 1 infinitely, just substitute the common ratio {bn} of the first term and the geometric series into the infinite geometric series.
⑶ Discuss whether n is odd or even. When n is odd, sum with the first n terms and formulas of geometric series, and when n is even, sum with the first n terms and formulas of arithmetic progression.
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I wish the landlord progress in his study o(∩_∩)o
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