已知 a∈Ra \in \mathbb{R} 且 a>1a > 1,利用代数方法求解不等式 ax−a+2(x−a)(1−x)≤1.\frac{ax - a + 2}{(x - a)(1 - x)} \le 1 . 由此求出满足 a∣x∣−a+2(∣x∣−a)(1−∣x∣)≤1\frac{a|x| - a + 2}{(|x| - a)(1 - |x|)} \le 1 的 xx 的取值集合。

  1. (i)

    已知 a∈Ra \in \mathbb{R} 且 a>1a > 1,利用代数方法求解不等式

    ax−a+2(x−a)(1−x)≤1.\frac{ax - a + 2}{(x - a)(1 - x)} \le 1 .

  2. (ii)

    由此求出满足 a∣x∣−a+2(∣x∣−a)(1−∣x∣)≤1\frac{a|x| - a + 2}{(|x| - a)(1 - |x|)} \le 1 的 xx 的取值集合。

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完整解法

3(i)

1. Rearrange the inequality to have 0 on one side and express it as a single algebraic fraction.

ax−a+2(x−a)(1−x)−1≤0\frac{ax - a + 2}{(x - a)(1 - x)} - 1 \le 0

ax−a+2−(x−a)(1−x)(x−a)(1−x)≤0\frac{ax - a + 2 - (x - a)(1 - x)}{(x - a)(1 - x)} \le 0

ax−a+2−(x−x2−a+ax)(x−a)(1−x)≤0\frac{ax - a + 2 - (x - x^2 - a + ax)}{(x - a)(1 - x)} \le 0

x2−x+2(x−a)(1−x)≤0\frac{x^2 - x + 2}{(x - a)(1 - x)} \le 0

2. Analyse the sign of the numerator.

Consider the numerator x2−x+2x^2 - x + 2:
Completing the square, we get:
x2−x+2=(x−12)2+74x^2 - x + 2 = \left(x - \frac{1}{2}\right)^2 + \frac{7}{4}
Since (x−12)2≥0\left(x - \frac{1}{2}\right)^2 \ge 0 for all real xx, it follows that x2−x+2>0x^2 - x + 2 > 0 for all real xx.
(Alternatively, the discriminant Δ=(−1)2−4(1)(2)=−7<0\Delta = (-1)^2 - 4(1)(2) = -7 < 0, and since the coefficient of x2x^2 is positive, the quadratic is strictly positive.)

3. Deduce the required inequality for the denominator and solve it.

Since the numerator is always strictly positive, for the whole fraction to be ≤0\le 0, the denominator must be strictly negative (note that the denominator cannot be equal to 00 as division by zero is undefined).
(x−a)(1−x)<0(x - a)(1 - x) < 0
Multiplying both sides by −1-1 reverses the inequality sign:
(x−a)(x−1)>0(x - a)(x - 1) > 0
Given that a>1a > 1, the critical values are 11 and aa, and they are ordered 1<a1 < a. Solving this quadratic inequality yields:
x<1 or x>ax < 1 \text{ or } x > a

答案

x<1 or x>ax < 1 \text{ or } x > a

3(ii)

1. Concisely solve the inequality from part (i) to establish the base result.

ax−a+2(x−a)(1−x)−1≤0\frac{ax - a + 2}{(x - a)(1 - x)} - 1 \le 0\nax−a+2−(−x2+x+ax−a)(x−a)(1−x)≤0\frac{ax - a + 2 - (-x^2 + x + ax - a)}{(x - a)(1 - x)} \le 0\nx2−x+2(x−a)(1−x)≤0\frac{x^2 - x + 2}{(x - a)(1 - x)} \le 0\nSince x2−x+2=(x−12)2+74>0x^2 - x + 2 = \left(x - \frac{1}{2}\right)^2 + \frac{7}{4} > 0 for all real xx, we require the denominator to be negative:\n(x−a)(1−x)<0  ⟹  (x−a)(x−1)>0(x - a)(1 - x) < 0 \implies (x - a)(x - 1) > 0\nGiven a>1a > 1, the solution to part (i) is x<1 or x>ax < 1 \text{ or } x > a.

2. Apply the substitution $x \to |x|$ to the result of part (i).

Comparing the target inequality with the one in part (i), we can substitute xx with ∣x∣|x| in the solution set:\n∣x∣<1or∣x∣>a|x| < 1 \quad \text{or} \quad |x| > a

3. Solve the resulting modulus inequalities to find the final range of $x$.

For ∣x∣<1|x| < 1:\n−1<x<1-1 < x < 1\nFor ∣x∣>a|x| > a, since we are given a>1>0a > 1 > 0:\nx<−aorx>ax < -a \quad \text{or} \quad x > a\nCombining these disjoint intervals, the set of values of xx is:\nx<−aor−1<x<1orx>ax < -a \quad \text{or} \quad -1 < x < 1 \quad \text{or} \quad x > a

答案

x<−a or −1<x<1 or x>ax < -a \text{ or } -1 < x < 1 \text{ or } x > a

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