Question

Use the identity $(a+b)(a-b)=a^2-b^2$ to evaluate:
$21\times 19$.

• A. $389$
• B. $399$
• C. $289$
• D. $429$

Answer 1

The correct option is B $399$

We know, $21\times 19=(20+1)\times (20-1)$ .
Applying the formula $(a+b)(a-b)=a^2-b^2$, where $a=20,b=1$,

we get,
$21\times 19=(20+1)\times (20-1)={20}^2-1^2=400-1=399$ .
Therefore, option $B$ is correct.