![]() Here we can see that this factor gets closer and closer to 1 for increasingly larger values of n. If the common ratio r of an infinite geometric sequence is a fraction where | r | < 1 (that is − 1 < r < 1), then the factor ( 1 − r n ) found in the formula for the nth partial sum tends toward 1 as n increases. For example, to calculate the sum of the first 15 terms of the geometric sequence defined by a n = 3 n + 1, use the formula with a 1 = 9 and r = 3. ![]() ![]() In other words, the nth partial sum of any geometric sequence can be calculated using the first term and the common ratio. S n − r S n = a 1 − a 1 r n S n ( 1 − r ) = a 1 ( 1 − r n )Īssuming r ≠ 1 dividing both sides by ( 1 − r ) leads us to the formula for the nth partial sum of a geometric sequence The sum of the first n terms of a geometric sequence, given by the formula: S n = a 1 ( 1 − r n ) 1 − r, r ≠ 1. Subtracting these two equations we then obtain, R S n = a 1 r + a 1 r 2 + a 1 r 3 + … + a 1 r n Multiplying both sides by r we can write, S n = a 1 + a 1 r + a 1 r 2 + … + a 1 r n − 1 Therefore, we next develop a formula that can be used to calculate the sum of the first n terms of any geometric sequence. However, the task of adding a large number of terms is not. For example, the sum of the first 5 terms of the geometric sequence defined by a n = 3 n + 1 follows: is the sum of the terms of a geometric sequence. In fact, any general term that is exponential in n is a geometric sequence.Ī geometric series The sum of the terms of a geometric sequence. In general, given the first term a 1 and the common ratio r of a geometric sequence we can write the following:Ī 2 = r a 1 a 3 = r a 2 = r ( a 1 r ) = a 1 r 2 a 4 = r a 3 = r ( a 1 r 2 ) = a 1 r 3 a 5 = r a 3 = r ( a 1 r 3 ) = a 1 r 4 ⋮įrom this we see that any geometric sequence can be written in terms of its first element, its common ratio, and the index as follows:Ī n = a 1 r n − 1 G e o m e t r i c S e q u e n c e Here a 1 = 9 and the ratio between any two successive terms is 3. For example, the following is a geometric sequence, Zeno’s paradox questions the conclusion of a geometric sequence, which paradoxically questions Atalanta’s ability to complete her walk to the end of the path! Our brain battles the fact that the sequence is infinite against our observable experience – of course Atalanta can walk to the end of the path! A related paradox to ponder: when would you say that the perimeter of a nested triangle in Problem #24 is equal to zero? This question might seem absurd, just like Zeno’s Paradox! Use your own thoughts to contemplate the question and debate your conclusion with a logical argument.A 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. Before traveling a quarter, she must travel one-eighth before an eighth, one-sixteenth and so on. Before she can get halfway there, she must get a quarter of the way there. Before she can get there, she must get halfway there. Suppose Atalanta wishes to walk to the end of a path.
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