Question

Use the identity $(a+b)(a-b)=a^2-b^2$ to evaluate:
$8.3\times 7.7$.

• A. $63.91$
• B. $63.81$
• C. $64.91$
• D. $64.21$

Answer 1

The correct option is A $63.91$

We are given the formula $(a+b)(a-b)=a^2-b^2$.

We know, $8.3\times 7.7=(8+0.3)(8-0.3)$,

where $a=8$ and $b=0.3$.

Now, solve as shown below:

$8.3\times 7.7$

$=(8+0.3)(8-0.3)=8^2-(0.3{)}^2\Rightarrow 8.3\times 7.7=64-0.09\Rightarrow 8.3\times 7.7=\mathrm{63.91.}$

Hence, $8.3\times 7.7=63.91$.

Therefore, option $A$ is correct.