(1) Since the series {an} is a arithmetic progression, the sum of the first n terms of the series {an} is required. We only need to find the first term, the last term and the number of terms and substitute them into the formula of the sum of the first n terms of arithmetic progression.

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.

// -

// -

I wish the landlord progress in his study o(∩_∩)o

Ask for adoption ~ ~ $ _ $