For example, the following is a geometric sequence, S 100 = 100 ( a 1 + a 100 ) 2 = 100 ( 1 + 199 ) 2 = 10,000Ī geometric sequence A sequence of numbers where each successive number is the product of the previous number and some constant r., or geometric progression Used when referring to a geometric sequence., is a sequence of numbers where each successive number is the product of the previous number and some constant r.Ī n = r a n − 1 G e o m e t i c S e q u e n c eĪnd because a n a n − 1 = r, the constant factor r is called the common ratio The constant r that is obtained from dividing any two successive terms of a geometric sequence a n a n − 1 = r. Use this formula to calculate the sum of the first 100 terms of the sequence defined by a n = 2 n − 1. S n = a n + ( a n − d ) + ( a n − 2 d ) + … + a 1Īnd adding these two equations together, the terms involving d add to zero and we obtain n factors of a 1 + a n:Ģ S n = ( a 1 + a n ) + ( a 1 + a n ) + … + ( a n + a 1 ) 2 S n = n ( a 1 + a n )ĭividing both sides by 2 leads us the formula for the nth partial sum of an arithmetic sequence The sum of the first n terms of an arithmetic sequence given by the formula: S n = n ( a 1 + a n ) 2. Therefore, we next develop a formula that can be used to calculate the sum of the first n terms, denoted S n, of any arithmetic sequence. ![]() However, consider adding the first 100 positive odd integers. S 5 = Σ n = 1 5 ( 2 n − 1 ) = + + + + = 1 + 3 + 5 + 7 + 9 = 25Īdding 5 positive odd integers, as we have done above, is managable. For example, the sum of the first 5 terms of the sequence defined by a n = 2 n − 1 follows: is the sum of the terms of an arithmetic sequence. In some cases, the first term of an arithmetic sequence may not be given.Īn arithmetic series The sum of the terms of an arithmetic sequence. For example, the following equation with domain a r i t h m e t i c m e a n s a 7 = 3 ( 7 ) − 11 = 21 − 11 = 10 is a function whose domain is a set of consecutive natural numbers beginning with 1. ![]() ![]() A sequence A function whose domain is a set of consecutive natural numbers starting with 1.
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