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A 6-ft tall man is walking away from a 15-ft post at a rate of 5 ft/sec in a straight line. how fast is the tip of his shadow moving when he is 40 ft away from the post?

Sagot :

Answer:

20

3

f

t

s

Explanation:

in this diagram, x is the distance from the man to the pole, and y is the distance from the tip of the man's shadow to the pole. i assume the man and pole are standing straight up, which means the 2 triangles are similar.

by similarity,

y

x

y

=

6

15

15

(

y

x

)

=

6

y

15

y

15

x

=

6

y

9

y

=

15

x

y

=

5

3

x

differentiate both sides with respect to

t

or time.

d

y

d

t

=

5

3

d

x

d

t

you know

d

x

d

t

=

4

f

t

s

because the man is walking that speed away from the pole. you want to find

d

y

d

t

, how fast the tip of the shadow is moving.

that means

d

y

d

t

=

5

3

4

f

t

s

=

20

3

f

t

s

actually, the man's distance from the pole doesn't matter since only his speed affects how fast his shadow moves