Q5.  In an arithmetic series the sum of the first, second and fourth terms are denoted by 𝑺𝟏, 𝑺𝟐, 𝒂𝒏𝒅 𝑺𝟒 respectively. Now 𝑺𝟏, 𝑺𝟐, 𝒂𝒏𝒅 𝑺𝟒 form three consecutive terms of a geometric sequence. Find the common ratio for this geometric sequence.*

1.5

3

4

2

None of the above

To solve this problem, let's break it down step by step:

### Step 1: Define the terms of the arithmetic sequence.
Let the first term of the arithmetic sequence be \( a \) and the common difference be \( d \).

- The first term \( T_1 = a \).
- The second term \( T_2 = a + d \).
- The fourth term \( T_4 = a + 3d \).

### Step 2: Define the sums \( S_1 \), \( S_2 \), and \( S_4 \).
- \( S_1 = T_1 = a \)
- \( S_2 = T_1 + T_2 = a + (a + d) = 2a + d \)
- \( S_4 = T_1 + T_2 + T_4 = a + (a + d) + (a + 3d) = 3a + 4d \)

### Step 3: Set up the geometric sequence condition.
Since \( S_1 \), \( S_2 \), and \( S_4 \) form a geometric sequence, the common ratio \( r \) should satisfy the following relationship:

\[
\frac{S_2}{S_1} = \frac{S_4}{S_2}
\]

Substituting the expressions we found:

\[
\frac{2a + d}{a} = \frac{3a + 4d}{2a + d}
\]

### Step 4: Solve the equation for the common ratio \( r \).
Cross multiply and simplify:

\[
(2a + d)^2 = a(3a + 4d)
\]

Expanding both sides:

\[
4a^2 + 4ad + d^2 = 3a^2 + 4ad
\]

Simplifying:

\[
4a^2 + d^2 = 3a^2
\]

This simplifies to:

\[
a^2 = d^2
\]

Thus:

\[
a = d \text{ or } a = -d
\]

### Step 5: Determine the common ratio \( r \).
- If \( a = d \), substitute \( d = a \) back into the expressions for \( S_1 \), \( S_2 \), and \( S_4 \) and find \( r \).
- If \( a = -d \), substitute \( d = -a \) back into the expressions for \( S_1 \), \( S_2 \), and \( S_4 \) and find \( r \).

Let's now calculate both cases.For the given problem:

- **Case 1 (where \( a = d \)):** The common ratio \( r \) is **3**.
- **Case 2 (where \( a = -d \)):** The common ratio \( r \) is **1**, but this does not match the options provided.

So, the correct answer based on the calculations is **3**.

Would you like further details or have any questions? Here are some related questions for practice:

1. How would the problem change if the arithmetic sequence started with a different first term?
2. What happens if the geometric sequence condition doesn't hold? What does it imply about the arithmetic sequence?
3. How would the solution change if you were given \( S_3 \) instead of \( S_4 \)?
4. How do different values of \( a \) and \( d \) affect the common ratio in the geometric sequence?
5. Can the common ratio ever be negative in this context? Why or why not?

**Tip:** In sequences, always double-check if the terms you derive from one type of sequence (like arithmetic) can form another type (like geometric) under the given conditions.

Learn how to find the common ratio in a geometric sequence using sums of terms from an arithmetic sequence. This problem involves arithmetic and geometric sequences and requires understanding of sequence properties and ratios. Suitable for high school students.

Math Problem Statement

Solution

Ask a new question for Free

By Image

Math Problem Analysis

Mathematical Concepts

Formulas

Theorems

Suitable Grade Level

Related Recommendation