Disproof by Counter Example (OCR A Level Maths A) : Revision Note

Disproof by Counter Example

What is disproof by counter-example?

• Disproof by counter-examples involves finding a value that does not work for the given statement

  • That value is called a counter-example

How do I disprove a result?

• You only need to find one value that does “not work”

• Numbers that have unusual results are often helpful to try

• Look carefully to see what types of number the result is claimed for – it may just be integers rather than real numbers

• Irrational numbers can be unusual

• Think about areas of maths where unusual things happen – such as in trigonometry where there are several angles with the same sine value