Chapter 8: Problem 26
Complete the following for the recursively defined sequence. (a) Find the first four terms. (b) Graph these terms. \(a_{n}=3 a_{n-1}^{2} ; a_{1}=2\)
Short Answer
Expert verified
The first four terms are 2, 12, 432, and 559872.
Step by step solution
01
Understand the Recursive Formula
The recursive sequence is defined by \( a_{n} = 3a_{n-1}^2 \) with the initial condition \( a_1 = 2 \). This means each term is computed based on the previous term.
02
Compute the First Term
Since we are given that \( a_1 = 2 \), the first term of the sequence is \( 2 \).
03
Compute the Second Term
Use the recursive formula to find the second term: \( a_2 = 3 \times (a_1)^2 = 3 \times 2^2 = 3 \times 4 = 12 \).
04
Compute the Third Term
Use the recursive formula to find the third term: \( a_3 = 3 \times (a_2)^2 = 3 \times 12^2 = 3 \times 144 = 432 \).
05
Compute the Fourth Term
Use the recursive formula to find the fourth term: \( a_4 = 3 \times (a_3)^2 = 3 \times 432^2 = 3 \times 186624 = 559872 \).
06
List the First Four Terms
The first four terms of the sequence are \( a_1 = 2 \), \( a_2 = 12 \), \( a_3 = 432 \), and \( a_4 = 559872 \).
07
Graph the Terms
Create a graph with the x-axis representing the term index \( n \) and the y-axis representing the term value \( a_n \). Plot the points (1, 2), (2, 12), (3, 432), and (4, 559872). You'll notice the graph increases very steeply.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Algebra
Algebra is a branch of mathematics dealing with symbols and the rules for manipulating those symbols. In the context of recursive sequences, algebra helps us express terms and relationships in a sequence with formulas. When we examine the sequence \( a_n = 3a_{n-1}^2 \), we are essentially using algebra to set up a pattern that describes how each term in the sequence is derived from the previous one. This requires basic algebraic manipulation like squaring terms and multiplying by constants.
- Expression: A mathematical phrase that can contain numbers, variables, and operation symbols.
- Term: Each element in a sequence or equation, like \( a_1, a_2, a_3 \), etc.
- Formula: An equation that calculates a new term based on known information.
Sequence Graphing
Sequence graphing involves representing terms of a sequence as points on a coordinate system. In our exercise, we graphed the sequence defined by \( a_n = 3a_{n-1}^2 \) and started with \( a_1 = 2 \). When we plot the terms \((1, 2), (2, 12), (3, 432), \) and \((4, 559872)\), each point corresponds to a term index \( n \) and its value \( a_n \).Graphing is insightful because:
- Visually shows the pattern: As seen, terms increase rapidly, revealing exponential growth.
- Identify trends: For recursive sequences, graphing can highlight how dramatically terms can change.
- Scientific tools: Graphs are essential for predicting future terms or understanding complex sequences.
Term Calculation
Term calculation in a recursive sequence involves determining each term based on the preceding one, using a given recursive formula. Let’s break down the calculation of terms in our exercise sequence.1. **Initial Term**: The sequence starts with an initial term, \( a_1 = 2 \).2. **Use of Recursive Formula**: For each subsequent term, we plug the previous term into the equation \( a_n = 3a_{n-1}^2 \): - Second term: \( a_2 = 3 \times a_1^2 = 3 \times 4 = 12 \) - Third term: \( a_3 = 3 \times a_2^2 = 3 \times 144 = 432 \) - Fourth term: \( a_4 = 3 \times a_3^2 = 3 \times 186624 = 559872 \)These steps underscore how each term depends strictly on its predecessor, showing the essence of recursive sequences.
Recursion
Recursion refers to a process where a function, or in our case, a sequence, is defined in terms of itself. This concept is central to defining recursive sequences like \( a_n = 3a_{n-1}^2 \). Understanding recursion involves grasping:
- Base case (starting point): Here, \( a_1 = 2 \) provides the initial term of our sequence.
- Recursive case (formula): Every new term builds on the result of its predecessor through the formula \( a_n = 3a_{n-1}^2 \).