What is it
Pythagoras' theorem states that in a right-angled triangle, the square of the hypotenuse (the longest side) equals the sum of the squares of the other two sides: a² + b² = c².
Why it matters
Pythagoras is one of the most widely used results in all of mathematics — from finding distances between coordinates to 3D geometry problems — and it's a guaranteed topic on every GCSE maths paper.
Exam tip
Always identify the hypotenuse first (it's opposite the right angle) before substituting into the formula — using the wrong side as the hypotenuse is the single most common error.
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