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Arithmetic Progressions

Question
CBSEENMA10007404

In which of the following situational, does the list of numbers involved make an arithmetic progression, and why?

The amount of air present in a cylinder when a vacuum pump removes 1 fourth of the air remaining in the cylinder at a time.

Solution

Amount of air present in the cylinder = x units (say) = a1
Amount of air present in the cylinder after one time removal of air by the vacuum pump
equals straight x plus straight x over 4 equals fraction numerator 3 straight x over denominator 4 end fraction space units space equals space straight a subscript 2
Amount of air present in the cylinder after two times removal of air by the vacuum pump
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and so on,
straight a subscript 1 minus straight a subscript 1 equals fraction numerator 3 straight x over denominator 4 end fraction minus straight x equals straight x over 4 space units
straight a subscript 3 minus straight a subscript 2 equals open parentheses 3 over 4 close parentheses squared straight x minus 3 over 4 straight x equals fraction numerator 3 straight x over denominator 4 end fraction space units.
As a3 – a7 ≠ a3 – a2, this list of numbers does not form an A .P.