calculus problem?

Question

Use Differentiation to solve the problem

1. A man starts walking north at 4ft/s from a point P. Five mins later a woman starts walking south at 5ft/s from a point 500 ft due east of P. At what rate are the people moving apart 15 min after the woman starts walking?

Answer 1

20 min = 1200 s
15 min = 900 s
1200 s * 4 ft/s = 4800 ft
900 s * 5 ft/s = 4500 ft
4800 ft + 4500 ft = 9300 ft
4 ft/s + 5 ft/s = 9 ft/s

Woman is 500 ft east and 9300 ft south of man.

cos (arctg (500/9300)) = 0.998558
9 ft/s * 0.998558 = 8.987021 ft/s

They are moving apart 8.987021 ft/s.
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