Pregunta
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find the \( n \) the term of the Series \( 2,-5,8,-11,14, \ldots \)

Ask by Salazar Donnelly. in Ghana
Feb 13,2025

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The nth term of the series is \( a_n = (-1)^{n+1} \times (3n - 1) \).

Solución

We are given the series: 2, -5, 8, -11, 14, … Step 1. Look at the absolute values of the terms: 2, 5, 8, 11, 14, … Notice that these form an arithmetic sequence with a first term of 2 and a common difference of 3. Thus, the nth term of the absolute values can be written as:  Absolute term = 2 + (n - 1) · 3 = 3n - 1. Step 2. Determine the signs: Observe that the first term is positive, the second term is negative, the third is positive, etc. This indicates that the sign alternates with each term. A common way to represent this alternating sign is to use (-1) raised to a power that depends on n. Here, using (-1)^(n+1) will yield a positive sign for odd n and a negative sign for even n. Step 3. Combine the information: Thus, the nth term of the series is:  aₙ = (-1)^(n+1)·(3n - 1). This formula satisfies all given terms:  For n = 1: a₁ = (-1)^(1+1)·(3·1 - 1) = 1·(2) = 2  For n = 2: a₂ = (-1)^(2+1)·(3·2 - 1) = (-1)·(5) = -5  For n = 3: a₃ = (-1)^(3+1)·(3·3 - 1) = 1·(8) = 8 Therefore, the nth term of the series is: aₙ = (-1)^(n+1)(3n - 1).

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To find the \( n \)th term of the series \( 2, -5, 8, -11, 14, \ldots \), we first observe that the series alternates in sign and increases in absolute value. The positive terms are \( 2, 8, 14, \ldots \) and the negative terms are \( -5, -11, \ldots \). The positive terms form an arithmetic sequence where the first term is 2 and the common difference is 6. The \( n \)th positive term can be expressed as: - For odd \( n \): \( 2 + 6\frac{(n-1)}{2} = 6m - 4 \) if \( n \) is \( 2m - 1 \). The negative terms are \( -5, -11, \ldots \) and follow a similar pattern. The first negative term can be expressed as: - For even \( n \): \( -5 - 6\frac{(n-2)}{2} = -6m + 1 \) if \( n \) is \( 2m \). In a single formula, we combine both cases: \[ a_n = \begin{cases} 6 \left(\frac{n - 1}{2}\right) + 2 & \text{if } n \text{ is odd} \\ -6 \left(\frac{n}{2}\right) + 1 & \text{if } n \text{ is even} \end{cases} \] Thus, the term \( a_n \) alternates based on whether \( n \) is odd or even.

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