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I don't know how to do related rates with the correct "derivative with respect to time" when the variables are not constant.

A girl flies a kite at a height of 300 feet, the wind carrying the kite horizontally away from her at a rate of 25 ft/sec. How fast must she let out the string when the kite is 500 feet from her?

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    when the variables are not constant. that cracked me up!2012-11-16
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    ... I blame my teacher. Ha, I mean when the variables move so I can't plug them into the basic equation. Does that sound smarter now?2012-11-16
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    $300^2+y^2=r^2$ where $y$ is horizontal distance and $r$ the length of the string. So $2yy'=2rr'$ and plug in what you know.2012-11-16

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