Math Problem Statement

The first three terms of a sequence are given. Write your answer as a decimal or whole number. Round to the nearest thousandth (if necessary). 3, comma, 5, comma, start fraction, 25, divided by, 3, end fraction, comma, point, point, . 3,5, 3 25 ​ ,... Find the 9th term.

Solution

Let's analyze the given sequence to identify the pattern and find the 9th term.

The first three terms of the sequence are:

  1. a1=3a_1 = 3
  2. a2=5a_2 = 5
  3. a3=253a_3 = \frac{25}{3}

To find the pattern, let's check if there's a consistent way the terms are progressing. A common approach is to check if the sequence might be arithmetic or geometric.

Checking if the sequence is arithmetic:

An arithmetic sequence has a constant difference between consecutive terms. Let's check the differences:

  1. Difference between the 2nd and 1st term: 53=25 - 3 = 2

  2. Difference between the 3rd and 2nd term: 2535=253153=1033.333\frac{25}{3} - 5 = \frac{25}{3} - \frac{15}{3} = \frac{10}{3} \approx 3.333

Since the differences are not equal, the sequence is not arithmetic.

Checking if the sequence is geometric:

A geometric sequence has a constant ratio between consecutive terms. Let's check the ratios:

  1. Ratio between the 2nd and 1st term: 531.6667\frac{5}{3} \approx 1.6667

  2. Ratio between the 3rd and 2nd term: 2535=253×15=2515=531.6667\frac{\frac{25}{3}}{5} = \frac{25}{3} \times \frac{1}{5} = \frac{25}{15} = \frac{5}{3} \approx 1.6667

Since the ratios are equal, the sequence is geometric with a common ratio r=53r = \frac{5}{3}.

Finding the 9th term:

The formula for the nnth term of a geometric sequence is: an=a1×rn1a_n = a_1 \times r^{n-1}

For n=9n = 9: a9=3×(53)8a_9 = 3 \times \left(\frac{5}{3}\right)^{8}

Let's calculate a9a_9:

a9=3×(53)8=3×(5838)a_9 = 3 \times \left(\frac{5}{3}\right)^{8} = 3 \times \left(\frac{5^8}{3^8}\right)

a9=3×5838=5837a_9 = \frac{3 \times 5^8}{3^8} = \frac{5^8}{3^7}

Now, compute 585^8 and 373^7:

58=390625and37=21875^8 = 390625 \quad \text{and} \quad 3^7 = 2187

a9=3906252187178.610a_9 = \frac{390625}{2187} \approx 178.610

Rounding to the nearest thousandth:

a9178.610a_9 \approx 178.610

Thus, the 9th term of the sequence is approximately 178.610.

Do you want further details or have any questions? Here are five related questions that might interest you:

  1. How do you determine whether a sequence is arithmetic or geometric?
  2. What is the significance of the common ratio in a geometric sequence?
  3. How do you calculate the sum of a geometric sequence?
  4. Can a sequence have both arithmetic and geometric properties?
  5. How does the choice of the common ratio affect the growth of a geometric sequence?

Tip: When dealing with sequences, identifying the pattern early on (arithmetic, geometric, etc.) makes it easier to predict future terms.

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Math Problem Analysis

Mathematical Concepts

Sequences
Geometric Sequences
Patterns in Numbers

Formulas

nth term formula of a geometric sequence: a_n = a_1 * r^(n-1)

Theorems

Geometric Sequence Theorem

Suitable Grade Level

Grades 9-12