Without using a calculator, solve the inequality $\dfrac{4x}{x-3} \ge 1. Hence find the range of values of xx that satisfies 4∣x∣∣x∣−3≥1\dfrac{4|x|}{|x|-3} \ge 1.

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Worked solution

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1. Rearrange the inequality to compare with zero and simplify into a single fraction.

4xx−3−1≥0\frac{4x}{x-3} - 1 \ge 0\n4x−(x−3)x−3≥0\frac{4x - (x - 3)}{x-3} \ge 0\n3x+3x−3≥0\frac{3x + 3}{x - 3} \ge 0

2. Identify critical values and determine the solution range for $x$.

The critical values are found by equating the numerator and denominator to zero:
3x+3=0  ⟹  x=−13x + 3 = 0 \implies x = -1
x−3=0  ⟹  x=3x - 3 = 0 \implies x = 3
Since the inequality is ≥0\ge 0 and x=3x=3 makes the fraction undefined, we test the intervals:

  • For x<−1x < -1, 3x+3x−3>0\frac{3x+3}{x-3} > 0
  • For −1<x<3-1 < x < 3, 3x+3x−3<0\frac{3x+3}{x-3} < 0
  • For x>3x > 3, 3x+3x−3>0\frac{3x+3}{x-3} > 0

Thus, the solution is x≤−1x \le -1 or x>3x > 3.

3. Use the result to solve the 'hence' part by substituting $x$ with $|x|$.

The inequality 4∣x∣∣x∣−3≥1\frac{4|x|}{|x|-3} \ge 1 is identical to the original inequality, with xx replaced by ∣x∣|x|.
Substituting ∣x∣|x| into our earlier solution gives:
∣x∣≤−1or∣x∣>3|x| \le -1 \quad \text{or} \quad |x| > 3

4. Evaluate the modulus inequalities to find the final range of values.

The inequality ∣x∣≤−1|x| \le -1 has no real solutions because the absolute value of a real number is always non-negative (∣x∣≥0|x| \ge 0).

For the second inequality:
∣x∣>3  ⟹  x<−3orx>3|x| > 3 \implies x < -3 \quad \text{or} \quad x > 3

Answer

x<−3 or x>3x < -3 \text{ or } x > 3

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