Question
Evaluate, using $(a + b)(a - b)= a^2 - b^2,399 \times 401$
✓
Answer
$399 \times 401$
$= (400 - 1) \times (400 + 1)$
$= (400)^2 - (1)^2$
$= 160000 - 1$
$= 159999.$
$= (400 - 1) \times (400 + 1)$
$= (400)^2 - (1)^2$
$= 160000 - 1$
$= 159999.$
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