What is 19 * 21?
What is 19 * 21?
Approach: Use (20 - 1)(20 + 1).
399. 400 - 1 = 399.
Follow-up: What is 29 * 31?
Key concepts: difference of squares.
What is 19 * 21?
Approach: Use (20 - 1)(20 + 1).
399. 400 - 1 = 399.
Follow-up: What is 29 * 31?
Key concepts: difference of squares.