/sci/ - Science & Math

How come in related rates problems involving triangles. (take the ladder falling against a wall problem) after we have found the rate at which it is falling and how much distance is on the ground the sides no longer add up to the original length of the ladder (hypotenuse)

https://www.youtube.com/watch?v=ps-r4nti5Go
the problem at 31:16 is what I am talking about

beforehand the original triangle has sides
13,5, and 12
but after 1 minute the new lengths are
13,12.2, and 9

im sure its something simple Im just missing but any explanation as to why would be great