Pythagoras' Theorem
For any 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
Worked example
A triangle has sides 3 cm and 4 cm meeting at a right angle. The hypotenuse is:
c = √(3² + 4²) = √(9 + 16) = √25 = 5 cm