Without using a calculator, solve

  1. (i)

    Solve

    1+6x≥x1 + \dfrac{6}{x} \ge x.

  2. (ii)

    Solve

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

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Worked solution

6(i)

1. Rearrange the inequality to form a single rational expression compared to zero.

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

2. Factorise the numerator to identify all critical values.

(x−3)(x+2)x≤0\frac{(x - 3)(x + 2)}{x} \le 0

The critical values are x=−2x = -2, x=0x = 0, and x=3x = 3.

3. Use a sign test to determine the intervals that satisfy the inequality, ensuring the denominator is not zero.

Testing the intervals separated by the critical values −2-2, 00, and 33 for the sign of (x−3)(x+2)x\frac{(x - 3)(x + 2)}{x}:

  • For x<−2x < -2: (−)(−)(−)<0\frac{(-)(-)}{(-)} < 0
  • For −2<x<0-2 < x < 0: (−)(+)(−)>0\frac{(-)(+)}{(-)} > 0
  • For 0<x<30 < x < 3: (−)(+)(+)<0\frac{(-)(+)}{(+)} < 0
  • For x>3x > 3: (+)(+)(+)>0\frac{(+)(+)}{(+)} > 0

Including the boundaries where the expression equals zero (but excluding x=0x=0 where it is undefined), we get:
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 rational inequality from part (i) to establish the base solution.

To solve part (ii) using the intended sequence, we first explicitly state the solution to part (i):
1+6x≥x1 + \frac{6}{x} \ge x
x+6−x2x≥0\frac{x + 6 - x^2}{x} \ge 0
(x−3)(x+2)x≤0\frac{(x-3)(x+2)}{x} \le 0
Using a sign diagram for the critical values −2-2, 00, and 33, the solution to (i) is:
x≤−2or0<x≤3x \le -2 \quad \text{or} \quad 0 < x \le 3

2. Substitute $x$ with $|x|$ using the relationship between the two inequalities and reject the invalid case.

The inequality in (ii) is identical to (i) but with xx replaced by ∣x∣|x|. We substitute ∣x∣|x| into the solution from (i):
∣x∣≤−2or0<∣x∣≤3|x| \le -2 \quad \text{or} \quad 0 < |x| \le 3
Since the absolute value of a real number is always non-negative (∣x∣≥0|x| \ge 0), the first inequality ∣x∣≤−2|x| \le -2 yields no real solutions.

3. Solve the valid modulus inequality to find the final range of values for $x$.

We solve the remaining inequality 0<∣x∣≤30 < |x| \le 3:
The condition ∣x∣>0|x| > 0 means x≠0x \neq 0.
The condition ∣x∣≤3|x| \le 3 gives −3≤x≤3-3 \le x \le 3.
Combining these results, the final solution is:
−3≤x<0or0<x≤3-3 \le x < 0 \quad \text{or} \quad 0 < x \le 3

Answer

−3≤x<0or0<x≤3-3 \le x < 0 \quad \text{or} \quad 0 < x \le 3

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