Pythagoras' theorem states that
a^2 + b^2 = c^2, where c is the hypotenuse.
To find the length of the hypotenuse, we must add together the squares of the other lengths, 18 and 80.
18^2 = 324
80^2 = 6400
c^2 = 324 + 6400 ---> Substitute these values in for a^2 and b^2
= 6724 ---> Square root this number to find c
c = 82
Therefore, the length of the hypotenuse is 82.