The legs of a right triangle are 18 centimeters and 80 centimeters long. what is the length of the hypotenuse

The hypotenuse is 82 centimeters long.

 The formula to find a hypotenuse is as follows:
a^2 + b^2 = c^2
Write out your triangle, labeling one part as 18, and another as 80. Connect a line between the end points of each line, and give a letter to a value. For my example, I will use 18 as a and 80 as b. 
18^2 + 80^2 = c^2
Solve this by squaring these numbers.
324 + 6400 = c^2
Add these numbers
6724 = c^2
 Then, finally, deprive the square root from 6724 and c^2
82 = c
 Since c is you hypotenuse, 82 centimeters is your answer. This checks out because your hypotenuse must be the largest number of the three, and if not, you did something wrong. However, this all checks out, meaning your hypotenuse is 82 centimeters. Hope this helped!