WAEC 2013 Mathematics · Question 45

Given that p13p^{\frac{1}{3}} = q3r\frac{\sqrt[3]{q}}{r}, make q the subject of the equation

Options

  1. A.q = pr\sqrt{r}
  2. B.q = p3r
  3. C.q = pr3
  4. D.q = pr13\frac{1}{3}

Answer: C. q = pr3

Explanation written with AI and checked against the answer key

Analyze the question: We are given p^(1/3) = (∛q)/r and must make q the subject.

Concept: To remove a cube root, cube both sides. Remember (a^(1/3))³ = a and (∛q)³ = q.

Solution:

  1. Start with: p^(1/3) = ∛q / r
  2. Multiply both sides by r: r·p^(1/3) = ∛q
  3. Cube both sides: (r·p^(1/3))³ = q
  4. Simplify: r³ · p = q

So q = pr³

Check: Take p = 8 and r = 2. Then p^(1/3) = 2, so ∛q / 2 = 2, which gives ∛q = 4 and q = 64. Using the formula: pr³ = 8 × 8 = 64 ✓.

Matching the options: The options appear to have lost their formatting. Option C, "pr3", is read as pr³, which matches our result. Option B, "p3r", would read as 3pr, which does not match.

Exam tip: When a variable sits inside a root, isolate the root first, then raise both sides to the power that cancels it (cube for a cube root, square for a square root). Also, when cubing a product, cube each factor: (r·p^(1/3))³ = r³·p.

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