Engineering Board Exam Reviewer · Related Rates (Time Rates) · medium
A kite is flying at a constant height of 60 ft and moving horizontally away from the person flying it at 5 ft/s. Find the rate at which the string is being paid out when 100 ft of string is out.
- 4 ft/s
- 5 ft/s
- 3 ft/s
- 6.25 ft/s
Answer
A. 4 ft/s
Solution
Let x = horizontal distance, L = string length: x2+3600=L2. At L=100, x=80. Differentiating: dL/dt=x/Ldx/dt=80/100(5)=4 ft/s.
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