Calculating squares

Try ( without a calculator )

21^2 ?

37^2 ?

If you need a calculator to calculate simple squares then you may need the following simple rule.

21^2 = 441

37^2 = 1369.

Or using ( x - k ) * ( x + k ) + k^2 = x^2

21^2 = 20 * 22 + 1^2 = 440 + 1 = 441

37^2 = 34 * 40 + 3^2 = 1200 + 160 + 9 = 1369.