Proof by Contradiction (Edexcel IAL Maths)

Spec point — 1.1

Proof by contradiction

What is proof by contradiction?

• A proof by contradiction assumes the opposite result is true
• Then, through a series of logical steps, shows that this cannot be so

How do I use proof by contradiction?

• This type of proof often involves explanation in words alongside mathematical statements

• Start with a statement assuming the opposite is true

  • e.g. 'Assume...'

• Use prime factorisation to write a composite number as a product of primes

Worked Example