# Geometric Sequences and Series

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Geometric Sequences and Series

What You Should Learn Recognize, write, and find the nth terms of geometric sequences. Find nth partial sums of geometric sequences. Find sums of infinite geometric series. Use geometric sequences to model and solve real-life problems.

Geometric Sequences

Geometric Sequences We know that a sequence whose consecutive terms have a common difference is an arithmetic sequence. In this section, you will study another important type of sequence called a geometric sequence. Consecutive terms of a geometric sequence have a common ratio.

Geometric Sequences

Example 1 – Examples of Geometric Sequences
a. The sequence whose nth term is 2n is geometric. For this sequence, the common ratio between consecutive terms is 2. 2, 4, 8, 16, , 2n, . . . b. The sequence whose nth term is 4(3n) is geometric. For this sequence, the common ratio between consecutive terms is 3. 12, 36, 108, 324, , 4(3n), . . . Begin with n = 1. Begin with n = 1.

Example 1 – Examples of Geometric Sequences
cont’d c. The sequence whose nth term is is geometric. For this sequence, the common ratio between consecutive terms is Begin with n = 1.

Geometric Sequences

The Sum of a Finite Geometric Sequence

The Sum of a Finite Geometric Sequence
The formula for the sum of a finite geometric sequence is as follows.

Example 6 – Finding the Sum of a Finite Geometric Sequence
Find the following sum. Solution: By writing out a few terms, you have

Example 6 – Solution Now, because a1 = 4(0.3), r = 0.3, and n = 12
cont’d Now, because a1 = 4(0.3), r = 0.3, and n = 12 you can apply the formula for the sum of a finite geometric sequence to obtain  1.71. Formula for sum of a finite geometric sequence Substitute 4(0.3) for a1, 0.3 for r, and 12 for n. Use a calculator.

Geometric Series

Geometric Series The sum of the terms of an infinite geometric sequence is called an infinite geometric series or simply a geometric series. The formula for the sum of a finite geometric sequence can, depending on the value of r, be extended to produce a formula for the sum of an infinite geometric series. Specifically, if the common ratio has the property that | r | < 1 then it can be shown that r n becomes arbitrarily close to zero as n increases without bound.

Geometric Series Consequently, This result is summarized as follows.

Example 7 – Finding the Sum of an Infinite Geometric Series
Find each sum. a. b Solution: = 10

Example 7 – Solution b. 3 + 0.3 + 0.03 + 0.003 + . . .
cont’d b = 3 + 3(0.1) + 3(0.1)2 + 3(0.2)  3.33