Correct Answer:
C. 39999
Correct answer: 39999. 201 × 199 = (200 + 1)(200 − 1) = 200² − 1 = 40000 − 1 = 39999.
Background: Use the identity (a + 1)(a − 1) = a² − 1.
- 2999: 2999 is far too small.
- 3999: 3999 is far too small.
- None of these: “None of these” is incorrect because 39999 is correct.
Did you know? It is quicker than long multiplication.
Memory trick: 200² − 1².