Question

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

• A. $(a+b)\times (a-b)=a^2-b^2$
• B. ${(a+b)}^2=a^2+2ab+b^2$
• C. ${(a-b)}^2=a^2-2ab+b^2$
• D. None of the above

Answer 1

The correct option is A $(a+b)\times (a-b)=a^2-b^2$

$1001\times 999=(1000+1)(1000-1)={1000}^2-1^2$

Using the identity,
$(a+b)\times (a-b)=a^2-b^2$
$=1000000-1=999999$