Free taster
5 of 6 sections open
What is Direct Proportion?
What is Direct Proportion?
If you buy 1 chocolate bar for £1, then 2 bars cost £2, 3 bars cost £3, and so on. The cost goes up at the SAME RATE as the number of bars - they're in direct proportion. When one quantity doubles, the other doubles too. The ratio between them stays constant!
Visual Guide
What does the symbol ∝ mean?: 'is proportional to'
y ∝ x means 'y is proportional to x'
This is equivalent to writing y = kx for some constant k.
Mathematics glossary
- What does the symbol ∝ mean?
- 'is proportional to' y ∝ x means 'y is proportional to x' This is equivalent to writing y = kx for some constant k.
Key Features of Direct Proportion
We write y ∝ x to mean "y is directly proportional to x"
Earn the mark scheme marks
Common Mistakes to Avoid
X Confusing direct and inverse proportion
Direct: both go up together. Inverse: one up, one down.
Check: does doubling x DOUBLE y or HALVE y?
X Forgetting the graph passes through (0,0)
Direct proportion ALWAYS passes through origin
If x = 0, then y = k × 0 = 0
Now try it yourself
Quiz · Question 1 of 15
Which of the following correctly reads the mathematical statement y ∝ x?
Tap an answer to check it
This topic in real past papers
Every real exam question we've found on direct proportion, with a full worked answer.
AQA Paper 3
Three of the four sittings we have full papers for state that one quantity is directly proportional to a power or root of another, then ask for the constant of proportionality and a further value to be found.
Edexcel Paper 1
All three sittings we have full papers for test proportion directly, and two of the three combine a direct and an inverse relationship into a single formula.