
Explanation:
We can apply the Pythagorean theorem here:

where x is the distance of the ladder base from the wall and y is the distance of the ladder top from the ground. Taking the time derivative of the expression above, we get

Solving for
we get

We can replace y by rearranging Eqn(1) such that

Therefore,

Since y is decreasing as the ladder is being lowered, we will assign a negative sign to
. Hence,

