1. (i)

    在不使用计算器的情况下,解不等式 3x2+12x2+3x−2>1\frac{3x^2 + 1}{2x^2 + 3x - 2} > 1。

  2. (ii)

    由此,解不等式 3x2+12x2+3∣x∣−2>1\frac{3x^2 + 1}{2x^2 + 3|x| - 2} > 1。

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完整解法

5(i)

1. Rearrange the inequality to have zero on one side and combine into a single algebraic fraction.

3x2+12x2+3x−2−1>0\frac{3x^2 + 1}{2x^2 + 3x - 2} - 1 > 0
3x2+1−(2x2+3x−2)2x2+3x−2>0\frac{3x^2 + 1 - (2x^2 + 3x - 2)}{2x^2 + 3x - 2} > 0
x2−3x+32x2+3x−2>0\frac{x^2 - 3x + 3}{2x^2 + 3x - 2} > 0

2. Analyse the sign of the numerator.

For the numerator x2−3x+3x^2 - 3x + 3:
Discriminant Δ=(−3)2−4(1)(3)=9−12=−3\Delta = (-3)^2 - 4(1)(3) = 9 - 12 = -3.
Since Δ<0\Delta < 0 and the coefficient of x2x^2 is positive (1>01 > 0), x2−3x+3>0x^2 - 3x + 3 > 0 for all real values of xx.

(Alternatively, by completing the square: x2−3x+3=(x−32)2+34>0x^2 - 3x + 3 = (x - \frac{3}{2})^2 + \frac{3}{4} > 0 for all xx.)

3. Deduce the condition for the denominator and solve the resulting quadratic inequality.

Since the numerator is strictly positive, the denominator must also be positive for the entire fraction to be greater than zero:
2x2+3x−2>02x^2 + 3x - 2 > 0
Factorising the quadratic expression:
(2x−1)(x+2)>0(2x - 1)(x + 2) > 0
Considering the roots x=12x = \frac{1}{2} and x=−2x = -2, the parabola opens upwards. Thus, the expression is positive outside the roots:
x<−2 or x>12x < -2 \text{ or } x > \frac{1}{2}

答案

x<−2 or x>12x < -2 \text{ or } x > \frac{1}{2}

5(ii)

1. Solve the inequality in part (i) to establish the necessary base result.

3x2+12x2+3x−2−1>0\frac{3x^2 + 1}{2x^2 + 3x - 2} - 1 > 0
x2−3x+3(2x−1)(x+2)>0\frac{x^2 - 3x + 3}{(2x - 1)(x + 2)} > 0
Since x2−3x+3=(x−32)2+34>0 for all real x,\text{Since } x^2 - 3x + 3 = \left(x - \frac{3}{2}\right)^2 + \frac{3}{4} > 0 \text{ for all real } x,
(2x−1)(x+2)>0(2x - 1)(x + 2) > 0
x<−2orx>12x < -2 \quad \text{or} \quad x > \frac{1}{2}

2. Relate the target inequality to part (i) by applying a modulus substitution.

Notice that x2=∣x∣2x^2 = |x|^2, so the inequality can be rewritten as:
3∣x∣2+12∣x∣2+3∣x∣−2>1\frac{3|x|^2 + 1}{2|x|^2 + 3|x| - 2} > 1
This is identical to the inequality in (i) with xx replaced by ∣x∣|x|. Using the established result:
∣x∣<−2or∣x∣>12|x| < -2 \quad \text{or} \quad |x| > \frac{1}{2}

3. Solve the resulting modulus inequalities, rejecting any invalid cases.

Since ∣x∣≥0|x| \ge 0 for all real xx, the inequality ∣x∣<−2|x| < -2 has no real solutions.
We only solve ∣x∣>12|x| > \frac{1}{2}:
x<−12orx>12x < -\frac{1}{2} \quad \text{or} \quad x > \frac{1}{2}

答案

x<−12x < -\frac{1}{2} or x>12x > \frac{1}{2}

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