Tuesday

September 2, 2014

September 2, 2014

Posted by **Anonymous** on Monday, March 14, 2011 at 1:47pm.

- Calculus -
**Reiny**, Monday, March 14, 2011 at 2:03pmlet the man be x ft from the lamppost.

At that time, let the length of his shadow be y ft

by ratios:

24/(x+y) = 6/y

24y = 6x+6y

18y=6x

3y=x

3dy/dt = dx/dt

but dx/dt = 2

dy/dt = 2/3 ft/s

so his shadow is increasing in length at 2/3 ft/sec,

notice that the end of his shadow would be MOVING at 2 +2/3 or 8/3 ft/se

also notice that the fact that he is 24 feet from the post does not enter the picture at all.

- Calculus -
**Anonymous**, Monday, March 14, 2011 at 2:08pmWhy is it "let the man be x ft from the lamppost. "

Why not 10 feet away?

& Why is it "also notice that the fact that he is 24 feet from the post does not enter the picture at all"

When the lamp post is 24 feet tall?

- Calculus -
**Reiny**, Monday, March 14, 2011 at 5:11pmI put the wrong number in when I made that last comment. It should have said " .... he is 10 feet from ..."

It has no effect on the solution.

We cannot use the 10 feet in the "general" case, the 10 feet is one specific instant.

in rate of change problems you <bnever use the specific case data until you have differentiated the equation.

**Answer this Question**

**Related Questions**

calculus - A person 150cm tall is walking away from a lamp post at the rate of ...

Calculus - (It's a related rate question) A man 6ft tall walks away from a lamp ...

math - A man is 6ft tall is standing 10 feet from the base of a lamp post. The ...

Calculus - A man is 6ft tall and is walking at night straight toward a lighted ...

math - A man 6 feet tall walks at a rate of 5 feet/sec toward a street lamp that...

Math 8R - HELP!!!!! - Mr. Martanarie bought a new lamp and lamppost for his home...

calculus - A 6ft tall woman is walking away from a srteet lamp at 4 ft/sec that ...

math - A girl 1.6 m tall, stands 3.6 m from a lamp post at night. Her shadow ...

Physics - A motorcycle is travelling at a constant acceleration in a straight ...

Trig/Precalc - a lamp post that is 8 feet high casts a shadow 5 feet long. how ...