Math Problem Statement

Given the sequences an = (2n-1)/(4n) + (n-1)/(4n) and bn = 2n * an, evaluate whether the following statements are true or false.

Solution

The question presented provides two sequences ana_n and bnb_n, defined as follows:

an=2n14n+n14na_n = \frac{2n - 1}{4n} + \frac{n - 1}{4n} bn=2nanb_n = 2n \cdot a_n

You are asked to determine whether the following six statements are true or false. Let's analyze the sequences step by step:

Step 1: Simplification of ana_n

First, simplify ana_n:

an=2n14n+n14n=2n1+n14n=3n24na_n = \frac{2n - 1}{4n} + \frac{n - 1}{4n} = \frac{2n - 1 + n - 1}{4n} = \frac{3n - 2}{4n}

Thus, an=3n24n=3412na_n = \frac{3n - 2}{4n} = \frac{3}{4} - \frac{1}{2n}

Step 2: Simplification of bnb_n

Now simplify bn=2nanb_n = 2n \cdot a_n:

bn=2n(3412n)=3n21b_n = 2n \cdot \left( \frac{3}{4} - \frac{1}{2n} \right) = \frac{3n}{2} - 1

Thus, we have the simplified forms: an=3412n,bn=3n21a_n = \frac{3}{4} - \frac{1}{2n}, \quad b_n = \frac{3n}{2} - 1

Analysis of the Statements:

  1. Convergence of ana_n:
    As nn \to \infty, the term 12n0\frac{1}{2n} \to 0, so an34a_n \to \frac{3}{4}. Hence, the sequence (an)n=1(a_n)_{n=1}^{\infty} converges.
    True.

  2. Boundedness of an34|a_n - \frac{3}{4}| for n30n \geq 30:
    From the simplified form of ana_n, we have an=3412na_n = \frac{3}{4} - \frac{1}{2n}. The absolute difference an34=12n|a_n - \frac{3}{4}| = \frac{1}{2n}, and for n30n \geq 30,
    12n160<150\frac{1}{2n} \leq \frac{1}{60} < \frac{1}{50} So the inequality holds.
    True.

  3. Monotonicity of ana_n:
    an=3412na_n = \frac{3}{4} - \frac{1}{2n} is a decreasing function of nn because 12n\frac{1}{2n} decreases as nn increases. Hence, (an)n=1(a_n)_{n=1}^{\infty} is not monotonically increasing.
    False.

  4. Boundedness of bnb_n:
    bn=3n21b_n = \frac{3n}{2} - 1, which grows without bound as nn \to \infty. Thus, (bn)n=1(b_n)_{n=1}^{\infty} is not bounded.
    False.

  5. Arithmetic progression of bnb_n:
    The general form of an arithmetic sequence is bn=a+(n1)db_n = a + (n-1)d. From the expression for bn=3n21b_n = \frac{3n}{2} - 1, it can be rewritten as:
    bn=32n1b_n = \frac{3}{2}n - 1 which is an arithmetic sequence with a common difference of 32\frac{3}{2}.
    True.

  6. Limit of ann\frac{a_n}{n}:
    Since an=3412na_n = \frac{3}{4} - \frac{1}{2n}, we analyze the limit:
    limnann=limn3412nn=limn(34n12n2)=0\lim_{n \to \infty} \frac{a_n}{n} = \lim_{n \to \infty} \frac{\frac{3}{4} - \frac{1}{2n}}{n} = \lim_{n \to \infty} \left( \frac{3}{4n} - \frac{1}{2n^2} \right) = 0 Hence, the limit is zero.
    True.

Summary of the Truth of the Statements:

  1. True
  2. True
  3. False
  4. False
  5. True
  6. True

Would you like more details on any step or clarification?

Here are 5 questions related to this analysis:

  1. How do you determine if a sequence converges?
  2. What is the geometric meaning of an arithmetic progression?
  3. Why is the sequence bnb_n unbounded?
  4. What is the significance of the limit limnann\lim_{n \to \infty} \frac{a_n}{n}?
  5. Can the sequence ana_n be increasing for any range of nn?

Tip: Always check both the limit behavior and the trend (monotonicity) when analyzing a sequence’s convergence and behavior over time.

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Math Problem Analysis

Mathematical Concepts

Sequences and Series
Convergence
Arithmetic Progression
Boundedness

Formulas

an = (2n-1)/(4n) + (n-1)/(4n)
bn = 2n * an
lim (n -> ∞) an
lim (n -> ∞) an / n

Theorems

Convergence Theorem
Boundedness Criterion
Arithmetic Sequence Properties

Suitable Grade Level

University Level (Mathematics, Calculus)