73-50=23 => 23x200=4600, 50-23=27, 27^2=729 => 73^2=4600+729=5329, all can be done in one’s head. For 27^2, one can just memorize, or: 27-25=2 => 2x100=200, 25

73-50=23 => 23x200=4600, 50-23=27, 27^2=729 => 73^2=4600+729=5329, all can be done in one’s head. For 27^2, one can just memorize, or: 27-25=2 => 2x100=200, 25-2=23, 23^2=529. 27^2=200+529=729. As long as one knows square of numbers up to 25, it is done for all up to 100 and more … :-).