To square a number ending in one; e.g. 21, 51, 121, ..., first square the prior number that is ending in zero, and then add both the prior number and the number to it.
Example 1:
To square 21 (21^2):
21: the number
20: the prior number
21^2 = 20^2 + 20 + 21 = 441
Example 2:
To square 61 (61^2):
61: the number
60: the prior number
61^2 = 60^2 + 60 + 61 = 3721
Example 3:
To square 121 (121^2):
121: the number
120: the prior number
121^2 = 120^2 + 120 + 121 = 14641
Example 1:
To square 21 (21^2):
21: the number
20: the prior number
21^2 = 20^2 + 20 + 21 = 441
Example 2:
To square 61 (61^2):
61: the number
60: the prior number
61^2 = 60^2 + 60 + 61 = 3721
Example 3:
To square 121 (121^2):
121: the number
120: the prior number
121^2 = 120^2 + 120 + 121 = 14641
