Pregunta

Consider the series \( \sum x^{2}(x-3)^{2} \). (1) For which values of \( x \) will this series converge? (2) If \( x=\frac{5}{2} \), calculate the sum to infinity of the series. (3) If the sum to infinity of the series is \( \frac{49}{2} \), determine the value of \( x \).

Ask by Rojas Bradley. in South Africa
Feb 03,2025

Solución de tutoría real

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The series converges for all real numbers \( x \). When \( x = \frac{5}{2} \), the sum to infinity of the series does not exist because it diverges. If the sum to infinity is \( \frac{49}{2} \), then \( x \) is approximately \( -1.183 \) or \( 4.183 \).

Solución

To analyze the series \( \sum x^{2}(x-3)^{2} \), we will address each part of the problem step by step. ### Part (1): Convergence of the Series The series can be expressed as: \[ \sum x^{2}(x-3)^{2} = \sum (x^{2}(x^{2} - 6x + 9)) = \sum (x^{4} - 6x^{3} + 9x^{2}) \] This is a polynomial series, and polynomial series converge for all real values of \( x \). Therefore, the series converges for all \( x \in \mathbb{R} \). ### Part (2): Sum to Infinity for \( x = \frac{5}{2} \) Substituting \( x = \frac{5}{2} \) into the series: \[ \sum \left( \left( \frac{5}{2} \right)^{2} \left( \frac{5}{2} - 3 \right)^{2} \right) \] Calculating \( \left( \frac{5}{2} \right)^{2} \): \[ \left( \frac{5}{2} \right)^{2} = \frac{25}{4} \] Calculating \( \left( \frac{5}{2} - 3 \right)^{2} \): \[ \frac{5}{2} - 3 = \frac{5}{2} - \frac{6}{2} = -\frac{1}{2} \] \[ \left( -\frac{1}{2} \right)^{2} = \frac{1}{4} \] Now, substituting these values back into the series: \[ \sum \left( \frac{25}{4} \cdot \frac{1}{4} \right) = \sum \frac{25}{16} \] Since this is a constant term, the sum diverges to infinity. Therefore, the sum to infinity of the series does not exist in the traditional sense, as it diverges. ### Part (3): Finding \( x \) for a Given Sum to Infinity If the sum to infinity of the series is given as \( \frac{49}{2} \), we set up the equation: \[ x^{2}(x-3)^{2} = \frac{49}{2} \] Expanding the left side: \[ x^{2}(x^{2} - 6x + 9) = \frac{49}{2} \] \[ x^{4} - 6x^{3} + 9x^{2} = \frac{49}{2} \] Multiplying through by 2 to eliminate the fraction: \[ 2x^{4} - 12x^{3} + 18x^{2} - 49 = 0 \] Now, we will solve this polynomial equation for \( x \). Let's use the tool to solve the equation \( 2x^{4} - 12x^{3} + 18x^{2} - 49 = 0 \). I'll proceed to solve this equation. Solve the equation(The complex numbers system) by following steps: - step0: Solve for \(x\) in the complex numbers system: \(2x^{4}-12x^{3}+18x^{2}-49=0\) - step1: Calculate: \(\begin{align}&x\approx 4.183235\\&x\approx \frac{3}{2}-1.643091i\\&x\approx \frac{3}{2}+1.643091i\\&x\approx -1.183235\end{align}\) - step2: Rewrite: \(x_{1}\approx -1.183235,x_{2}\approx \frac{3}{2}+1.643091i,x_{3}\approx \frac{3}{2}-1.643091i,x_{4}\approx 4.183235\) - step3: Remove the complex number(s): \(x_{1}\approx -1.183235,x_{2}\approx 4.183235\) The solutions to the equation \( 2x^{4} - 12x^{3} + 18x^{2} - 49 = 0 \) are approximately: 1. \( x_{1} \approx -1.183235 \) 2. \( x_{2} \approx 4.183235 \) ### Summary of Results: 1. The series converges for all \( x \in \mathbb{R} \). 2. The sum to infinity of the series does not exist as it diverges. 3. The values of \( x \) for which the sum to infinity equals \( \frac{49}{2} \) are approximately \( -1.183235 \) and \( 4.183235 \).

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The Deep Dive

To analyze the series \( \sum x^{2}(x-3)^{2} \), we can first rewrite the general term of the series: 1. **For convergence:** The term \( x^{2}(x-3)^{2} \) is a polynomial in \( x \), which does not converge to zero for any finite value of \( x \). Therefore, the series diverges for all \( x \). 2. **If \( x = \frac{5}{2} \):** The term becomes: \[ \left(\frac{5}{2}\right)^{2} \left(\frac{5}{2}-3\right)^{2} = \left(\frac{25}{4}\right) \left(-\frac{1}{2}\right)^{2} = \left(\frac{25}{4}\right) \left(\frac{1}{4}\right) = \frac{25}{16}. \] Since the series diverges, the sum to infinity cannot be calculated in the traditional sense. Hence, no numerical result can be provided. 3. **If the sum to infinity is \( \frac{49}{2} \):** Since earlier we established that the series diverges, it means that there is no such value of \( x \) for which the sum converges to \( \frac{49}{2} \) or any other finite number. In summary, the series \( \sum x^{2}(x-3)^{2} \) diverges for all \( x \), and thus the questions related to calculating the sum to infinity yield no values.

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