0Take a 2-digit number ending in 1.
Subtract 1 from the number.
Square the difference.
Add the difference twice to its square.
Add 1.
Example: 61 x 61
Subtract 1: 61 - 1 = 60
Square the difference: 60 × 60 = 3600
Add the Difference Twice to its Square: 3600 + 60 + 60 = 3720
Add 1:
3720 + 1 = 3721
There you have it! 61 × 61 = 3721
