CIE IGCSE Maths

Revision Notes

2.11.1 Direct & Inverse Proportion

What is proportion?

  • Proportion is a way of talking about how two variables are related to each other
  • Direct proportion: as one variable goes up the other goes up by the same factor
    eg. If one variable is multiplied by 3, so is the other
  • Inverse proportion: as one variable goes up the other goes down by the same factor
    eg. If one variable is multiplied by 3, the other is divided by 3

You should know what the graphs look like:

Direct & Inverse Proportion RN, downloadable IGCSE & GCSE Maths revision notes

How do we deal with proportion questions?

  1. Identify the two VARIABLES (called A and B below)
  2. Choose TYPE of proportion:
    A is DIRECTLY proportional to B use formula A = kB
    A is INVERSELY proportional to B use formula A = k÷B
  3. FIND k using the values given in the question
  4. Write FORMULA for A in terms of B (using your value of k)
  5. USE formula to find the required quantity

Exam Tip

Even if the question doesn’t ask for a formula (Step 4. above) it is always worth working one out and using it in all but the simplest cases.

Worked Example

Direct & Inverse Proportion Example, IGCSE & GCSE Maths revision notes

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