# Math Help - related rates

1. ## related rates

A person flying a kite holds the string 5 feet above ground level, and the string is payed out at a rate of 2 ft/sec as the kite moves horizontally at an altitude of 105 feet. Assuming there is no sag in the string, find the rate at which the kite is moving when 125 feet of string has been payed out.

Thanks, I'm hopeless at these problems.

Let D be the distance from John to kite. Then horizontal distance is
$x= \sqrt{D^2-100^2}$
Differentiate this with respect to t
$\frac{dx}{dt} = \frac{D}{\sqrt{D^2-100^2}} \frac{dD}{dt}$
Plug the values in and that's it