1. ## Related Rates, shadow and streetlight problem

I need some help setting up this related rates problem.

A man 6 ft tall is walking at what rate of 3 ft/s toward a streetlight 18 ft high. At what rate is his shadow length changing?
I know this problem is about using similar triangles but I'm not quite sure where to start. Can someone just give me a basic equation to start with? I've looked at similar problems but they all do things differently. :|

2. Originally Posted by nautica17
I need some help setting up this related rates problem.

I know this problem is about using similar triangles but I'm not quite sure where to start. Can someone just give me a basic equation to start with? I've looked at similar problems but they all do things differently. :|
Here is a sketch of the diagram

Using simialr triangles you get

$\displaystyle \frac{18}{6}=\frac{x+y}{y} \iff y=\frac{1}{2}x$

This should get you started.

3. Originally Posted by TheEmptySet
Here is a sketch of the diagram

Using simialr triangles you get

$\displaystyle \frac{18}{6}=\frac{x+y}{y} \iff y=\frac{1}{2}x$

This should get you started.
Thank-you!