Calculus differentiation?

Question

A person 6 feet tall walks away form a streetlight at the rate of 5ft/s. If the light is 18 feet above ground level, how fast is the person's shadow lengthening?

Answer 1

Start by calculating the length of his shadow as a function of his distance from the streetlight.

That's not even a matter of trigonometry -- just Euclidean geometry (similar triangles).

Then, if necessary, use the Chain Rule.