# Thread: Calc Problem I need help!

1. ## Calc Problem I need help!

1. a boy is flying a kite at a height 150 feet. if the kite moves horizontally away from the boy at a rate of 20 feet per second, how fast is the string being let out when the kite is 250 feet from him?

2. Originally Posted by BeckyK
1. a boy is flying a kite at a height 150 feet. if the kite moves horizontally away from the boy at a rate of 20 feet per second, how fast is the string being let out when the kite is 250 feet from him?
sketch a picture ...

let x = kite's horizontal distance
y = kite's vertical distance
z = kite's straight line distance

dx/dt = 20 ft/s
since y = a constant 150 ft, dy/dt = 0

the question is asking for dz/dt when z = 250 ft.

set up the relationship between the variables using Pythagoras, take the derivative w/r to time, then evaluate dz/dt.

3. Originally Posted by BeckyK
1. a boy is flying a kite at a height 150 feet. if the kite moves horizontally away from the boy at a rate of 20 feet per second, how fast is the string being let out when the kite is 250 feet from him?
See here

The numbers are different but the problem is otherwise the same and the same method applies.

RonL