How to square a 2-digit number ending with 1

41²

Subtract 1 from 41 to get 40.

Then add 1 and twice 40 to the square of 40.

40² + 40 + 40 + 1 = 1600 + 80 + 1 = 1681

Example #11

Find 61²

61 = 60² + 60 + 60 + 1 = 3600 + 120 + 1 = 3721