NCERT Solutions
Class 10 Maths
Arithmetic Progressions

Ex. 5.1 Q.1
In which of the following situations, does the list of numbers involved make an arithmetic progression, and why?
- The taxi fare after each km when the fare is Rs 15 for the first km and Rs 8 for each additional km.
- The amount of air present in a cylinder when a vacuum pump removes 1/4 of the air remaining in the cylinder at a time.
- 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.
- The amount of money in the account every year, when Rs 10000 is deposited at compound interest at 8 % per annum.
- It can be observed that
Taxi fare for 1 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.
- Let the initial volume of air in a cylinder be V lit. In each stroke, the vacuum pump removes 1/4 of air remaining in the cylinder at a time. In other words, after every stroke, only 1- 1/4 =
3/4 part of air will remain.
Therefore, volumes will be V, 3V/4, (3/4)2V, (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.
- 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.
- We know that if Rs P is deposited at r% compound interest per annum for n years, our money will be P(1 + r/100)n after n years.
Therefore, after every year, our money will be
10000(1 + 8/100), 10000(1 + 8/100)2, 10000(1 + 8/100)3, ……………
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.