Chapter 8: Problem 138
The value of \(2^{1 / 4} \cdot 4^{1 / 8} \cdot 8^{1 / 16} \ldots \infty\) is: \(\quad[2002]\) (A) 1 (B) 2 (C) \(3 / 2\) (D) 4
Short Answer
Expert verified
The value is 2.
Step by step solution
01
Understand the Problem
We are required to find the value of the infinite product: \(2^{1/4} \cdot 4^{1/8} \cdot 8^{1/16} \ldots \infty\). This sequence consists of powers of even numbers with decreasing exponents.
02
Express Terms Using Base 2
Note that each number in the sequence can be expressed as a power of 2. Specifically: \(4 = 2^2\), \(8 = 2^3\), and so forth. Therefore, the sequence can be rewritten as: \(2^{1/4} \cdot (2^2)^{1/8} \cdot (2^3)^{1/16} \cdots\)
03
Simplify Exponents
Distribute the exponents through each factor, giving us \(2^{1/4} \cdot 2^{2/8} \cdot 2^{3/16} \cdots\). Further simplifying, \(2^{2/8} = 2^{1/4}\) and \(2^{3/16}\) can be calculated similarly.
04
Recognize the Pattern
Notice that each exponent is following a pattern: \(\frac{1}{4}, \frac{2}{8}, \frac{3}{16}, \ldots\). In general, the \(n\)-th term can be represented as \(\frac{n}{2^{n+1}}\). This is a geometric series in summation form.
05
Sum the Infinite Series
The exponents to find the total exponent of the power of 2 would be \(\sum_{n=1}^\infty \frac{n}{2^{n+1}}\). Recognize that this series can be rearranged by factoring out \(1/2\) and becomes \(\frac{1}{2} \sum_{n=1}^\infty \frac{n}{2^n}\), a known series with sum \(2\). Therefore, the net exponent becomes \(\frac{1}{2} \times 2 = 1\).
06
Calculate the Value of the Expression
As the exponent of 2 is 1, the value of the expression is \(2^1 = 2\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Geometric Series
A geometric series is a series with a constant ratio between successive terms. It is an important concept in mathematics that is often used to describe and solve problems involving repeated multiplication by a constant factor. In general, a geometric series can be written as:
- a, ar, ar^2, ar^3, ..., ar^n, ...
- where \(a\) is the first term and \(r\) is the common ratio.
Exponents
Exponents are used to express repeated multiplication of a number by itself. For example, \(2^3 = 2 \times 2 \times 2 = 8\). In the context of this exercise, every term in the sequence is a power with a decreasing exponent. To work more easily with exponents:
- Remember the rules of exponents such as \(a^m \times a^n = a^{m+n}\) and \((a^m)^n = a^{m \times n}\).
- Be comfortable converting different bases into the same base where possible, making calculations simpler.
Series Summation
Series summation involves adding up a sequence of numbers. In mathematics, when dealing with series, especially infinite ones, it is important to determine if the series converges to a finite value.
- A series converges if the sum of its terms approaches a fixed number as you add more and more terms.
- Many known series, like the geometric series, have specific formulas for easy summation.