This study notes covers Worked Example 2 (Higher): Proportional to Square within Proportion Graphs for GCSE Mathematics. Revise Proportion Graphs in Ratio & Proportion for GCSE Mathematics with 14 exam-style questions and 4 flashcards. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 5 of 6 in this topic. Use this study notes to connect the idea to the wider topic before moving on to questions and flashcards.
Worked Example 2 (Higher): Proportional to Square
y is directly proportional to x². When x = 3, y = 45. Find y when x = 5.
Step 1 Set up the equation
y ∝ x² means y = kx²
Step 2 Find k
45 = k × 3² = k × 9
k = 45 ÷ 9 = 5
Step 3 Find y when x = 5
y = 5 × 5² = 5 × 25 = 125
Practice questions for Proportion Graphs
Which of the following describes the graph of a direct proportion relationship?
Aisha says: 'Because the graph of y against x is a straight line, y must be directly proportional to x.' Is Aisha correct? Explain your answer.