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