Number ends with 1 can be do by this.

Square - double - 1

Example - 21² = 441

2² = 4

Double of 2 = 4

1 = 1

31² = 961

3² = 9

Double of 3 = 6

1 = 1

41² = 1681

4² = 16

Double of 4 = 8

1 = 1

51² = 2601

5² = 25

Double of 5 = 10 (carry 1 )

1 = 1

71² = 5041

7² = 49

Double of 7 = 14 (carry 1 )

1 = 1

Well , I always do like this (not take a single sec to do)

**51² = 26 01**

26 = 5² + 1

01 = reverse digit of 5’s double

**61² = 37 21**

37 = 6² + 1

21 = reverse digit of 6’s double

**71² = 5041**

50 = 7² + 1

41 = reverse digit of 7’s double

**81² = 6561**

65 = 8² + 1

61 = reverse digit of 8’s double

**91² = 8281**

82 = 9² + 1

81 = reverse digit of 9’s double

Note : I am using this only when numbers double is 2 digit no

Like 5’s double = 10

If you not wanna use this you can use first one, applicable to all numbers.