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.