Related rates of change

Chas3down

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?

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

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Homework Helper

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?

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?

Skins
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 ?