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Related rates of change

  • Thread starter Chas3down
  • Start date
  • #1
60
0

Homework Statement


A street light is at the top of a pole that is 18 feet tall. A woman 6 ft tall walks away from the pole with a speed of 4 ft/sec along a straight path. How fast is the length of her shadow moving when she is 35 ft from the base of the pole?


Homework Equations



a^2 + b^2 = c^2
b' = 4


The Attempt at a Solution



18/6 relate to
x/(x-35)

I got x = 35/2

so total length of side is 105/2

18^2 + 105/2 ^ 2 = 111/2 ^ 2

2aa' + 2bb' = 2cc'

a' = 0, height of pole never changes

2*35*4 = 111/2 * c'

c' = 420/111

but wrong anwser
 
Last edited:

Answers and Replies

  • #2
Dick
Science Advisor
Homework Helper
26,258
618

Homework Statement


A street light is at the top of a pole that is 18 feet tall. A woman 6 ft tall walks away from the pole with a speed of 4 ft/sec along a straight path. How fast is the length of her shadow moving when she is 35 ft from the base of the pole?


Homework Equations



a^2 + b^2 = c^2
b' = 4


The Attempt at a Solution



18/6 relate to
x/(x-35)

I got x = 35/2

so total length of side is 105/2

18^2 + 105/2 ^ 2 = 111/2 ^ 2

2aa' + 2bb' = 2cc'

a' = 0, height of pole never changes

2*35*4 = 111/2 * c'

c' = 420/111

but wrong anwser
You should at least explain what your variables mean. Could you do that?
 
  • #3
74
4
Are you looking for the rate at which the shadow is moving relative to the person or relative to the base of the light pole ?
 

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