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 May 6th, 2009, 02:35 PM #1 Member   Joined: May 2009 Posts: 58 Thanks: 0 Related rates 1. A man 6 ft tall walks at rate of 200 feet per minute towards a street light which is 18 ft above the ground. At what rate is the tip of his shadow moving? Answer : let shadow of street light = h and dh/dt = rate of change in street light shadow let shadow of man to the street light= r and rate of change of the tip of his shadow = dr/dt The man and the street light form two similar triangles which gives: height of street light : height of man = shadow of street light : shadow of man 6 : 18 = r-h : r 1:3 = r-h : r r=3r-3h 2r=3h differentiate each with respect to time t 2dr/dt = 3dh/t 2dr/dt = 3 (-200) dr/dt = -300 ft/minute (I'm not sure whether it should be -300 or +300)
 May 6th, 2009, 02:37 PM #2 Guest   Joined: Posts: n/a Thanks: Re: Related rates This was answered previously: viewtopic.php?f=15&t=7826

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