Math Problem Statement

Find the first three terms in the expansion of (1 - 2x)^8 in ascending powers of x, where |x| < 1. Hence, find the coefficient of x^2 in the expansion of (7 + 3x)/√(1 - 2x).

Solution

Let's solve the two parts of the problem shown in the image.

Part (i)

We need to find the first three terms in the expansion of (12x)8(1 - 2x)^8 in ascending powers of xx.

To expand (12x)8(1 - 2x)^8, we use the Binomial Theorem:

(a+b)n=k=0n(nk)ankbk(a + b)^n = \sum_{k=0}^{n} \binom{n}{k} a^{n-k} b^k

Here, a=1a = 1, b=2xb = -2x, and n=8n = 8. The first three terms of the expansion will be for k=0k = 0, k=1k = 1, and k=2k = 2.

  1. Term for k=0k = 0: (80)180(2x)0=111=1\binom{8}{0} \cdot 1^{8-0} \cdot (-2x)^0 = 1 \cdot 1 \cdot 1 = 1

  2. Term for k=1k = 1: (81)181(2x)1=81(2x)=16x\binom{8}{1} \cdot 1^{8-1} \cdot (-2x)^1 = 8 \cdot 1 \cdot (-2x) = -16x

  3. Term for k=2k = 2: (82)182(2x)2=8×72×114x2=284x2=112x2\binom{8}{2} \cdot 1^{8-2} \cdot (-2x)^2 = \frac{8 \times 7}{2 \times 1} \cdot 1 \cdot 4x^2 = 28 \cdot 4x^2 = 112x^2

Thus, the first three terms in the expansion of (12x)8(1 - 2x)^8 are: 116x+112x21 - 16x + 112x^2

Part (ii)

Now, we need to find the coefficient of x2x^2 in the expansion of the expression:

7+3x12x\frac{7 + 3x}{\sqrt{1 - 2x}}

First, express 12x\sqrt{1 - 2x} as a binomial series. Using the Binomial Theorem for negative exponents:

(12x)12=k=0(12k)(2x)k(1 - 2x)^{-\frac{1}{2}} = \sum_{k=0}^{\infty} \binom{-\frac{1}{2}}{k} (-2x)^k

The first few terms of the expansion are: (12x)12=1+(12)(2x)+12322!(2x)2+(1 - 2x)^{-\frac{1}{2}} = 1 + \left(-\frac{1}{2}\right)(-2x) + \frac{-\frac{1}{2} \cdot -\frac{3}{2}}{2!}(-2x)^2 + \dots =1+x+3x22+= 1 + x + \frac{3x^2}{2} + \dots

Now, consider the product: 7+3x12x=(7+3x)(1+x+3x22+)\frac{7 + 3x}{\sqrt{1 - 2x}} = (7 + 3x)(1 + x + \frac{3x^2}{2} + \dots)

Expanding and collecting the terms up to x2x^2:

= 7 + 7x + \frac{21x^2}{2} + 3x + 3x^2$$ $$= 7 + 10x + \left(\frac{21}{2} + 3\right)x^2 = 7 + 10x + \frac{27x^2}{2}$$ Therefore, the coefficient of $$x^2$$ is $$\frac{27}{2}$$. --- Would you like more details or have any questions? Here are five related questions you might find useful: 1. How do we expand $$(1 + x)^n$$ for large values of $$n$$? 2. How does the Binomial Theorem change for negative and fractional exponents? 3. What is the general form of the Taylor series expansion for $$\sqrt{1 - x}$$? 4. How would the expansion change if the exponent in $$\sqrt{1 - 2x}$$ was $$-\frac{3}{2}$$? 5. Can you explain the convergence criteria for the series expansions used here? **Tip:** When expanding functions involving radicals, remember that the series may converge only for certain values of $$x$$, typically $$|x| < 1$$.

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Math Problem Analysis

Mathematical Concepts

Binomial Theorem
Series Expansion
Algebra

Formulas

Binomial expansion formula: (a + b)^n = Σ (n choose k) * a^(n-k) * b^k
Binomial expansion for negative exponents

Theorems

Binomial Theorem

Suitable Grade Level

Grades 11-12