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QUESTION 2 Consider the series: \( \sum_{k=0}^{\infty} x^{2}(x-3)^{k} \) 2.1.1. For which values of x will this series converge? 2.1.2 If \( x=\frac{5}{2} \), calculate the sum to infinity of the series. 2.1.3. Determine the value of x if the sum to infinity is \( \frac{49}{2} \).

Ask by Fleming Vargas. in South Africa
Feb 04,2025

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The series \( \sum_{k=0}^{\infty} x^{2}(x-3)^{k} \) converges for all real numbers \( x \) where \( 2 < x < 4 \).

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### **Question 2.1.1** **For which values of \( x \) will the series \( \sum_{k=0}^{\infty} x^{2}(x-3)^{k} \) converge?** The given series is: \[ \sum_{k=0}^{\infty} x^{2}(x-3)^{k} \] This can be rewritten as: \[ x^{2} \sum_{k=0}^{\infty} (x-3)^{k} \] The inner sum is a geometric series with the common ratio \( r = x - 3 \). **Condition for Convergence of a Geometric Series:** A geometric series \( \sum_{k=0}^{\infty} r^{k} \) converges if and only if: \[ |r| < 1 \] Applying this condition to our series: \[ |x - 3| < 1 \] Solving the inequality: \[ -1 < x - 3 < 1 \] Add 3 to all parts of the inequality: \[ 2 < x < 4 \] **Conclusion:** The series \( \sum_{k=0}^{\infty} x^{2}(x-3)^{k} \) converges for all real numbers \( x \) such that: \[ 2 < x < 4 \] --- **Answer:** Problem 2.1.1 Answer: All real numbers x with x larger than 2 and less than 4. In other words, the series converges for 2 < x < 4.

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To determine the convergence of the series \( \sum_{k=0}^{\infty} x^{2}(x-3)^{k} \), we recognize it as a geometric series where the common ratio is \( (x-3) \). For a geometric series to converge, the absolute value of the common ratio must be less than 1, leading to the condition \( |x-3| < 1 \). Therefore, the series converges for \( 2 < x < 4 \). Now, substituting \( x = \frac{5}{2} \) into the series, we have \( \sum_{k=0}^{\infty} \left(\frac{5}{2}\right)^{2} \left(\frac{5}{2} - 3\right)^{k} = \sum_{k=0}^{\infty} \frac{25}{4} \left(-\frac{1}{2}\right)^{k} \). The first term is \( \frac{25}{4} \) and the common ratio \( r = -\frac{1}{2} \), and since \( |r| < 1 \), we can sum the series using the formula \( S = \frac{a}{1 - r} \), leading to \( S = \frac{\frac{25}{4}}{1 - (-\frac{1}{2})} = \frac{\frac{25}{4}}{\frac{3}{2}} = \frac{25}{6} \). To find \( x \) such that the sum to infinity is \( \frac{49}{2} \), we set up the equation \( \frac{x^{2}}{1 - (x-3)} = \frac{49}{2} \). This simplifies to \( \frac{x^{2}}{4 - x} = \frac{49}{2} \). Cross-multiplying gives \( 2x^{2} = 49(4 - x) \), leading to \( 2x^{2} + 49x - 196 = 0 \). Solving this quadratic equation via the quadratic formula will yield the values of \( x \) for which the sum is \( \frac{49}{2} \).

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