Posted by AH on Friday, November 2, 2012 at 1:06pm.
make a sketch
label the man's distance from the post as x
and the length of his shadow y
by similar triangles and ratios
6/y = 15/(x+y)
15y = 6x + 6y
9y = 6x
3y = 2x
3dy/dt = 2dx/dt
dy/dt = (2/3)(7) = 14/3 ft/s
so the length of his shadow is lengthening at 14/3 ft/s
but it is moving at 14/3 + 7 or 35/7 ft/sec
Thank you very much!
By the way, 14/3+7 equals 35/3, not 35/7.
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