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2. Sum formula of arithmetic sequence:

If the first term of arithmetic progression is and the last term is, then the arithmetic progression summation expression is:

That is (the first item+the last item) × the number of items ÷2.

Geometric series of extended data refers to a series in which the ratio of each term to its previous term is equal to the same constant from the second term, usually expressed by G and P. This constant is called the common ratio of geometric series, usually expressed by the letter Q (q≠0) and geometric series a 1≠ 0. Where each item in {an} is not 0. Note: When q= 1, an is a constant series.

Definition of geometric series:

Arithmetic progression refers to the sequence of the same constant whose difference between each term and the previous term is equal to the second term, usually expressed by a and p ... This constant is called arithmetic progression's tolerance, often expressed by the letter D.

References:

Baidu Encyclopedia-Geometric Series Baidu Encyclopedia-Arithmetic Series