Squaring 2 digit number ending in 9

1. Choose a 2-digit number ending in 9.
2. The last digit of the answer is always 1: _ _ _ 1
3. Multiply the first digit by 8 and add 8 (keep the carry): _ _ X _
4. Multiply the first digit by the next consecutive number and add the carry: the product is the first two digits: XX _ _.

Example:

1. If the number is 39:
2. The last digit of the answer is 1: _ _ _ 1
3. Multiply the first digit (3) by 8 and add 8 (keep the carry): 8 × 3 = 24, 24 + 8 = 32; the next digit of the answer is 2 (keep carry 3): _ _ 2 1
4. Multiply the first digit (3) by the next number (4) and add the carry (3): 3 × 4 = 12, 12 + 3 = 15 (the first two digits): 1 5 _ _
5. So 39 × 39 = 1521.