Derivative Word Problem: spotlight on tightrope

I

Icy

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Jan 1, 2008
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A tightrope is stretched 30 feet above the ground between the Jay and the Tee buildings, which are 50 feet apart. A tightrope walker, walking at a constant rate of 2 feet per second from point A to point B, is illuminated by a sportlight 70 feet above point A.

calcxy2.jpg


A. How fast is the shadow of the tightrope walker's feet moving along the ground when she is midway between the buildings.

There are other parts but I think if I can get help with this part I can figure out the others.

All I know is that I need to find the derivative of the spot indicated by the red circle. Granted I probably could have done this before winter break from school but now I feel like I forgot everything. So if anyone could help me with the first step or two I'd greatly appreciate it.
 
Re: Derivative Word Problem

Looking at similar triangles you see that \(\displaystyle \frac{x}{y} = \frac{{40}}{{70}}\).
You are given that \(\displaystyle dx=2\).
You want to find \(\displaystyle dy\).
 
Re: Derivative Word Problem

Thanks a lot. That should help me quite a bit.
 
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