This study notes covers Worked Example 5: Square Relationships within Proportion for GCSE Mathematics. Revise Proportion in Number for GCSE Mathematics with 12 exam-style questions and 22 flashcards. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 11 of 16 in this topic. Use this study notes to connect the idea to the wider topic before moving on to questions and flashcards.
Worked Example 5: Square Relationships
y is directly proportional to x². When x = 3, y = 18. Find y when x = 5.
Solution
y ∝ x², so y = kx²
When x = 3, y = 18: 18 = k × 3²
18 = k × 9
k = 18 ÷ 9 = 2
So y = 2x²
When x = 5: y = 2 × 5² = 2 × 25 = 50
Answer: y = 50
Practice questions for Proportion
y is directly proportional to x. Which equation could represent this relationship?
Explain how you can tell from a graph whether two quantities are in direct or inverse proportion.