I found a very similar problem on the internet that I know will help.
It's here and it's Example 6.
A street light is at the top of a 17 foot tall pole. A woman 6 ft tall walks away from the pole with a speed of 8 ft/s along a straight path. How fast is the tip of her shadow moving when she is 45 feet from the base of the pole?
Hello, keemariee!
With these "streetlight" problems, we must be very careful.
Sometimes they ask how fast the tip of the shadow is moving.
Sometimes they ask how fast how fast the shadow is growing.
A street light is at the top of a 17 foot tall pole.
A woman 6 ft tall walks away from the pole with a speed of 8 ft/s along a straight path.
How fast is the tip of her shadow moving when she is 45 feet from the base of the pole?Code:A * | * | * | * C 17 | * | | * | 6| * | | * B *---------------*---------------* E : y D x-y : : - - - - - - x - - - - - - - - :
The streetlight is:
The woman is:
Let
Let
We have: .
Differentiate with respect to time: .
Since
You don't know how to prove they are similar triangles, or how to get the equation?
I'll assume the equation. The two similar triangles are CDE and ABE. So if we take the ratio of any two sides of CDE you can write a "similar" expression for ABE. For example, look at the sides x - y and 6 in CDE. These are the horizontal "leg" and vertical leg of CDE. So we know that the ratio we want from ABE will involve the horizontal (x) and vertical legs.
Thus
-Dan