The problem asks us to find the coefficient of $x^4$ in the expansion of $(1-2x)^6$.

AlgebraBinomial TheoremPolynomial ExpansionCombinatoricsCoefficients
2025/4/19

1. Problem Description

The problem asks us to find the coefficient of x4x^4 in the expansion of (12x)6(1-2x)^6.

2. Solution Steps

We will use the binomial theorem to expand (12x)6(1-2x)^6. The binomial theorem states that for any non-negative integer nn and any real numbers aa and bb:
(a+b)n=k=0n(nk)ankbk(a+b)^n = \sum_{k=0}^{n} {n \choose k} a^{n-k} b^k
In our case, a=1a=1, b=2xb=-2x, and n=6n=6. We want the term with x4x^4, so we need k=4k=4. Thus, we have
(64)(1)64(2x)4=(64)(1)2(2x)4{6 \choose 4} (1)^{6-4} (-2x)^4 = {6 \choose 4} (1)^2 (-2x)^4
We first calculate the binomial coefficient:
(64)=6!4!(64)!=6!4!2!=6×5×4×3×2×1(4×3×2×1)(2×1)=6×52×1=15{6 \choose 4} = \frac{6!}{4! (6-4)!} = \frac{6!}{4! 2!} = \frac{6 \times 5 \times 4 \times 3 \times 2 \times 1}{(4 \times 3 \times 2 \times 1) (2 \times 1)} = \frac{6 \times 5}{2 \times 1} = 15
Next, we simplify the term:
(1)2=1(1)^2 = 1
(2x)4=(2)4x4=16x4(-2x)^4 = (-2)^4 x^4 = 16 x^4
Therefore, the term with x4x^4 is:
15×1×16x4=240x415 \times 1 \times 16x^4 = 240 x^4
The coefficient of x4x^4 is
2
4
0.

3. Final Answer

The coefficient of x4x^4 in the expression of (12x)6(1-2x)^6 is 240.

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