or 91^2=92*90+1=8280+1=8281

or 91^2 = (90+1)^2 = 8100 + 180 + 1. if the units digit is too high you can go the other route, i.e. (100-9)^2. William's way is better when (x+1)(x-1) yields at least one round number as the factor but if you get something like 43^2, 44*42 isn't much easier.

to willie: 43^2=40*46+9? :)

91^2 = (100 -9)(100-9) = 10,000 -2(900) + 81 = 9,881