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Quiz yourself on ProportionGet your flashcardsBuild my revision planTrack my progressHigher Level: 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
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