A girl flies a kite at a height of 300ft, the wind carrying the kite horizontally away from her at a rate of 25 ft/s. How fast must she let out the string when the kite is 500 ft away from her?
The correct answer is 20ft/s
Can anyone tell me what I did wrong?
Furthermore, you said "How fast must she let out the string when the kite is 500 ft away from her?"
when clearly the given answer requires "... the kite is  ft away from her?"
Using 400 ft and what I posted, the correct answer falls out in a couple of lines.
Just to post the solution...
You were right mr fantastic, it was indeed using that formula. For a moment I thought all related rates problems could only be solved using trig relationships, that's why I kind of "ignored" your solution. My apologies for that.
The problem can be solved with the values in this picture:
And taking the derivative with respect to t of both sides of the following equation:
Can be solved in an easier way by taking the derivative of both sides from the following equation and solving:
Even though I didn't use your solution because by that time I DIDN'T KNOW about the usage of the quadratic formula for solving these problems, I thank you for having tried to help. If I did know about it, then I would probably solved the problem using your solution.