Rate problem:please plase help me.
A spotlight on the ground shines on a wall 12m away. If a man 2m tall walks from the spotlight toward a building at a speed of 16ms, how fast is the length of his shadow on the building decreasing when he is 4m from the building?

Question

Rate problem:please plase help me.
A spotlight on the ground shines on a wall 12m away. If a man 2m tall walks from the spotlight toward a building at a speed of 16ms, how fast is the length of his shadow on the building decreasing when he is 4m from the building?

anonymous · Answer

@satellite73  please help

anonymous · Answer

@calmat01

anonymous · Answer

ok, hang on.brb.

anonymous · Answer

ok,

anonymous · Answer

Ok, most important thing in Related rates problems is to draw a picture to get a sense of what is being asked.  Have you drawn a picture?

anonymous · Answer

No i havnt.  It confused me from the first step

anonymous · Answer

Sorry, my laptop is giving me issues...it keeps freezing.  I will type it up and see if I can post it....give me a few as this thing is super slow!

anonymous · Answer

Haha its okay! thank you for taking your time

anonymous · Answer

I am almost done...give me a few more.

anonymous · Answer

its totally ok, thanks

anonymous · Answer

lol...I have it, you sure you don't want to see it?

anonymous · Answer

Heres's the picture.|dw:1363314443849:dw|

By similar triangles $$\frac{ y }{ 12 }=\frac{ 2 }{ 12-x}$$  Then just take the derivative of both sides WRT time.

anonymous · Answer

|dw:1363314756665:dw|Failed to mention y=length of shadow, and dx/dt=-16, we want dy/dt when x=4.   @Dodo1