What is Pythagoras' Theorem?

You can use pythagoras' theorem to find the length of a side of a right angled triangle, provided that at least two of the lengths of the sides are known. The general rule is a² + b² = c² where a, b and c are the labelled sides of the triangle with c being the hypotenuse (the longest side). Therefore if you have a triangle where the hypotenuse is unknown but the other two sides are 3 and 4 units long: 3² + 4² = c² 9 + 16 = c² c² = 25 c = 5 therefore the hypotenuse length is 5.