# Related Rates, shadow and streetlight problem

• Mar 5th 2010, 10:41 AM
nautica17
Related Rates, shadow and streetlight problem
I need some help setting up this related rates problem.

Quote:

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. :|
• Mar 5th 2010, 11:45 AM
TheEmptySet
Quote:

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

Attachment 15787

Using simialr triangles you get

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

This should get you started.
• Mar 5th 2010, 12:00 PM
nautica17
Quote:

Originally Posted by TheEmptySet
Here is a sketch of the diagram

Attachment 15787

Using simialr triangles you get

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

This should get you started.

Thank-you!