Question

Which of the following identity can be used to solve 1001 $\times$ 999 ?

• A. (a+b) ×(a-b)=a²-b²
• B. (a+b)²=a²+2ab+b²
• C. (a-b)²=a²-2ab+b²
• D. (x+y)³=x³+3x²y+3xy²+y³

Answer 1

The correct option is A

(a+b) ×(a-b)=a²-b²

$1001=(1000+1)$
$999=(1000-1)$
$(1000+1)(1000-1)={1000}^2-1^1$ (Identity $(a+b)\times (a-b)=a^2-b^2$ is used)
$=1000000-1$
$=999999$