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