Squaring Numbers Ending in Nine

To square a number ending in nine; e.g. 29, 59, 129, ..., first square the next number that is ending in zero, and then subtract both the number and the next number from it.


Example 1:

To square 29 (29^2):

29: the number
30: the next number

29^2 = 30^2 - (29 + 30) = 900 - 59 = 841


Example 2:

To square 69 (69^2):

69: the number
70: the next number

69^2 = 70^2 - (69 + 70) = 4900 - 139 = 4761


Example 3:

To square 129 (129^2):

129: the number
130: the next number

129^2 = 130^2 - (129 + 130) = 16900 - 259 = 16641

Squaring Numbers Ending in One

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