Q. Find whether each of the following numbers is a perfect square or not?
(iii) 81
If it is a sum of successive odd natural numbers starting with 1, then it is a perfect square.
So we try to express 81 in successive odd natural numbers. This can also be done by subtracting successive odd natural numbers from 81.
81 - 1 = 80
80 - 3 = 77
77 - 5 = 72
72 - 7 = 65
65 - 9 = 56
56 - 11 = 45
45 - 13 = 32
32 - 15 = 17
17 - 17 = 0
Thus 81 is a perfect square number.