Question

If $a+b=1$ and $a-b=7$, then find $ab$

• A. $-12$
• B. $12$
• C. $7$
• D. $1$

Answer 1

The correct option is A $-12$

Given, $a+b=1$
Squaring both sides, we get
$(a+b{)}^2=1^2$
$=>a^2+b^2+2ab=1$ ------------(i)
Also $a-b=7$
Squaring both sides, we get
$(a-b{)}^2=7^2$
$=>a^2+b^2-2ab=49$ ------------(ii)
(i) - (ii), we get
$=>(a^2+b^2+2ab)-(a^2+b^2+2ab)=1-49$
$=>4ab=-48$
$=>ab=-12$