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The greek philosopher zeno of elea presented a number of paradoxes that caused a great deal of concern to the mathematicians of his period.
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These paradoxes were important in the development of the notion of infinitesimals.
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One of the most famous is the racecourse paradox.
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It can be stated as follows.
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A runner can never reach the end of a race.
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As he runs the track, he must first run half the length of the track, then half the remaining distance, so that at this point he has run a half plus a quarter, which is three quarters of the length of the track, and a quarter remains to be run.
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After running half the remaining distance, he finds that he still has one eighth of the track left to run, and so on indefinitely.
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So we're going to set up the infinite series in summation form that gives the total distance covered by the runner trying to run a track that is three miles long.
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So first he has to run half the length of the track.
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So it's a three mile long track, so he first has to run three halves of a mile.
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Then he has to run half of the remaining distance.
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Well, if he's already run three halves of a mile, three halves of a mile remain.
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So he has to run half of that remaining distance, which is three quarters of a mile.
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Now then he has to run half of the remaining distance again.
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So there's three quarters remaining after he's run three halves plus three quarters, so he has to run another three eighths of a mile.
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And this pattern continues, plus three sixteenths, plus dot dot dot, forever and ever...