Ch. 8 - Sequences, Induction, and Probability

Blitzer8th EditionCollege AlgebraISBN: 9780136970514Not the one you use?Change textbook

Blitzer 8th Edition

Ch. 8 - Sequences, Induction, and Probability

Problem 25

Chapter 9, Problem 25

Use the formula for the sum of the first n terms of a geometric sequence to solve Exercises 25–30. Find the sum of the first 12 terms of the geometric sequence: 2, 6, 18, 54, ...

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Identify the first term $ a_1 $ of the geometric sequence. In this sequence, the first term is $ 2 $.

Determine the common ratio $ r $ by dividing the second term by the first term: $ r = \frac{6}{2} $.

Recall the formula for the sum of the first $ n $ terms of a geometric sequence: \[ S_n = a_1 \times \frac{1 - r^n}{1 - r} \] where $ n $ is the number of terms.

Substitute the values $ a_1 = 2 $, $ r $ (from step 2), and $ n = 12 $ into the formula to set up the expression for $ S_{12} $.

Simplify the expression by calculating $ r^{12} $, then compute the numerator and denominator separately before dividing to find the sum of the first 12 terms.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Geometric Sequence

A geometric sequence is a list of numbers where each term after the first is found by multiplying the previous term by a constant called the common ratio. For example, in the sequence 2, 6, 18, 54, the common ratio is 3 because each term is three times the previous one.

Sum of the First n Terms of a Geometric Sequence

The sum of the first n terms of a geometric sequence can be calculated using the formula S_n = a(1 - r^n) / (1 - r), where a is the first term, r is the common ratio, and n is the number of terms. This formula helps find the total sum without adding each term individually.

Exponents and Powers

Exponents represent repeated multiplication of a base number. In the sum formula, r^n means multiplying the common ratio r by itself n times. Understanding how to work with exponents is essential for correctly applying the sum formula and simplifying expressions.

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