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13–22 Show how to approximate the required work by a Riemann
sum. Then express the work as an integral and evaluate it.
13. A heavy rope, 50 ft long, weighs 0.5 lb/ft and hangs over the
edge of a building 120 ft high.
(a) How much work is done in pulling the rope to the top of
the building?
(b) How much work is done in pulling half the rope to the
top of the building?
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