Without using a calculator, solve

  1. (i)

    1+6x≥x1 + \frac{6}{x} \ge x,

  2. (ii)

    1+6∣x∣≥∣x∣1 + \frac{6}{|x|} \ge |x|.

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Worked solution

6(i)

1. Bring all terms to one side and combine them into a single rational expression.

1+6x−x≥01 + \frac{6}{x} - x \ge 0
x+6−x2x≥0\frac{x + 6 - x^2}{x} \ge 0

2. Factorise the numerator and identify the critical values.

−(x2−x−6)x≥0\frac{-(x^2 - x - 6)}{x} \ge 0
(x−3)(x+2)x≤0\frac{(x-3)(x+2)}{x} \le 0
Critical values are x=3,−2,0x = 3, -2, 0.

3. Perform a sign test to determine the valid intervals, noting the denominator restriction $x \neq 0$.

Testing intervals around the critical values for (x−3)(x+2)x≤0\frac{(x-3)(x+2)}{x} \le 0:

  • For x<−2x < -2, expression is (−)(−)(−)→negative≤0\frac{(-)(-)}{(-)} \rightarrow \text{negative} \le 0 (Valid)
  • For −2<x<0-2 < x < 0, expression is (−)(+)(−)→positive>0\frac{(-)(+)}{(-)} \rightarrow \text{positive} > 0 (Invalid)
  • For 0<x<30 < x < 3, expression is (−)(+)(+)→negative≤0\frac{(-)(+)}{(+)} \rightarrow \text{negative} \le 0 (Valid)
  • For x>3x > 3, expression is (+)(+)(+)→positive>0\frac{(+)(+)}{(+)} \rightarrow \text{positive} > 0 (Invalid)

Therefore, the solution is x≤−2 or 0<x≤3x \le -2 \text{ or } 0 < x \le 3.

Answer

x≤−2 or 0<x≤3x \le -2 \text{ or } 0 < x \le 3

6(ii)

1. Solve the inequality from part (i) to use as a prerequisite

To solve part (ii) efficiently, we first establish the solution to part (i):
1+6x−x≥01 + \frac{6}{x} - x \ge 0
x+6−x2x≥0\frac{x + 6 - x^2}{x} \ge 0
x2−x−6x≤0\frac{x^2 - x - 6}{x} \le 0
(x−3)(x+2)x≤0\frac{(x-3)(x+2)}{x} \le 0
Using a sign test with critical values x=−2,0,3x = -2, 0, 3, the solution to part (i) is:
x≤−2or0<x≤3x \le -2 \quad \text{or} \quad 0 < x \le 3

2. Substitute $x$ with $|x|$ based on the structure of the new inequality

The inequality in part (ii) is 1+6∣x∣≥∣x∣1 + \frac{6}{|x|} \ge |x|.
This is identical to the inequality in (i) with xx replaced by ∣x∣|x|.
Substituting x=∣x∣x = |x| into the solution from part (i) gives:
∣x∣≤−2or0<∣x∣≤3|x| \le -2 \quad \text{or} \quad 0 < |x| \le 3

3. Solve the resulting modulus inequalities

Analyse the two conditions separately:

  1. ∣x∣≤−2|x| \le -2 has no real solution because ∣x∣≥0|x| \ge 0 for all real xx.
  2. For the second condition 0<∣x∣≤30 < |x| \le 3:
    • ∣x∣>0  ⟹  x≠0|x| > 0 \implies x \neq 0
    • ∣x∣≤3  ⟹  −3≤x≤3|x| \le 3 \implies -3 \le x \le 3

Combining these constraints, we exclude x=0x = 0 from the interval [−3,3][-3, 3]:
−3≤x<0or0<x≤3-3 \le x < 0 \quad \text{or} \quad 0 < x \le 3

Answer

−3≤x<0 or 0<x≤3-3 \le x < 0 \text{ or } 0 < x \le 3

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