Question

In which of the following situations, does the list of numbers involved make as arithmetic progression and why?
(1) The taxi fare after each km when the fare is Rs 15 for the first km and Rs 8 for each additional km.
(2) The amount of air present in a cylinder when a vacuum pump removes \(\frac{1}{4}\) of the air remaining in the cylinder at a time.
(3) The cost of digging a well after every metre of digging, when it costs Rs 150 for the first metre and rises by Rs 50 for each subsequent metre.
(4) The amount of money in the account every year, when Rs 10000 is deposited at compound interest at 8% per annum.

Answer 1

(i) It can be observed that
Taxi fare for \(1^{st}\) km = 15
Taxi fare for first 2 km = 15 + 8 = 23
Taxi fare for first 3 km = 23 + 8 = 31
Taxi fare for first 4 km = 31 + 8 = 39
Clearly 15, 23, 31, 39 … forms an A.P. because every term is 8 more than the preceding term.

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(ii) Let the initial volume of air in a cylinder be V lit.
In each stroke, the vacuum pump removes \(\frac{1}{4}\) of air remaining in the cylinder at a time.
In other words, after every stroke, only \(1-\frac{1}{4}=\frac{3}{4}{th}\)  part of air will remain.
Therefore, volumes will be \(V\frac{3}{4}V,(\frac{3}{4})^2V,(\frac{3}{4})^3V\) ....
Clearly, it can be observed that the adjacent terms of this series do not have the same difference between them.
Therefore, this is not an A.P.

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(iii) Cost of digging for first metre = 150
Cost of digging for first 2 metres = 150 + 50 = 200
Cost of digging for first 3 metres = 200 + 50 = 250
Cost of digging for first 4 metres = 250 + 50 = 300
Clearly, 150, 200, 250, 300 … forms an A.P. because every term is 50 more than the preceding term.

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(iv) We know that if Rs P is deposited at r% compound interest per annum for n years, our money will be \(P (1 +\frac{r}{100})^n\) after n years.
Therefore, after every year, our money will be :
\(10000(1 + \frac{8}{100})\), \(10000 (1 + \frac{8}{100})^2\), \(10000 (1 + \frac{8}{100})^3 , 10000 (1 + \frac{8}{100})^4,\)......
Clearly, adjacent terms of this series do not have the same difference between them.
Therefore, this is not an A.P.