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(i) Write down the first three terms in the expansion, in ascending powers of \( x \), of \( \left(2-\frac{x}{8}\right)^{n} \), where \( n \) is a positive integer greater than 2 .

Ask by Grant Santiago. in Singapore
Mar 09,2025

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Answer

The first three terms of the expansion are: \[ 2^n - \frac{n\,2^{n-1}}{8}\,x + \binom{n}{2}\frac{2^{n-2}}{64}\,x^2. \]

Solution

The binomial expansion of \[ \left(2-\frac{x}{8}\right)^n \] is given by \[ \left(2-\frac{x}{8}\right)^n=\sum_{k=0}^{n}\binom{n}{k}\,2^{n-k}\left(-\frac{x}{8}\right)^k. \] For the first three terms (that is, for \( k=0, 1, 2 \)) we have: 1. When \( k=0 \): \[ \binom{n}{0}\,2^n\left(-\frac{x}{8}\right)^0 = 2^n. \] 2. When \( k=1 \): \[ \binom{n}{1}\,2^{n-1}\left(-\frac{x}{8}\right) = -\frac{n\,2^{n-1}}{8}\,x. \] 3. When \( k=2 \): \[ \binom{n}{2}\,2^{n-2}\left(-\frac{x}{8}\right)^2 = \binom{n}{2}\,2^{n-2}\frac{x^2}{64}. \] Thus, the first three terms of the expansion in ascending powers of \( x \) are: \[ 2^n - \frac{n\,2^{n-1}}{8}\,x + \binom{n}{2}\frac{2^{n-2}}{64}\,x^2. \]

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To find the first three terms in the expansion of \( \left(2 - \frac{x}{8}\right)^n \), we can use the Binomial Theorem, which states that: \[ (a + b)^n = \sum_{k=0}^{n} \binom{n}{k} a^{n-k} b^k \] Here, let \( a = 2 \) and \( b = -\frac{x}{8} \). The first three terms (for \( k = 0, 1, 2 \)) will be: 1. For \( k = 0 \): \[ \binom{n}{0} (2)^{n} (-\frac{x}{8})^0 = 2^n \] 2. For \( k = 1 \): \[ \binom{n}{1} (2)^{n-1} (-\frac{x}{8})^{1} = n \cdot 2^{n-1} \cdot \left(-\frac{x}{8}\right) = -\frac{n \cdot 2^{n-1} x}{8} \] 3. For \( k = 2 \): \[ \binom{n}{2} (2)^{n-2} (-\frac{x}{8})^{2} = \frac{n(n-1)}{2} \cdot 2^{n-2} \cdot \left(-\frac{x}{8}\right)^{2} = \frac{n(n-1) \cdot 2^{n-2} x^2}{64} \] Combining these, the first three terms in ascending powers of \( x \) are: \[ 2^n - \frac{n \cdot 2^{n-1} x}{8} + \frac{n(n-1) \cdot 2^{n-2} x^2}{64} \]

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