Question

If $a+b+c=5$ and $a^2+b^2+c^2=29$, then the value of $ab+bc+ca$ is

• A. $-2$
• B. $-6$
• C. $-8$
• D. $6$

Answer 1

The correct option is A $-2$

$a+b+c=5$ $a^2+b^2+c^2=29$

$(a+b+c{)}^2=a^2+b^2+c^2+2ab+2bc+2ca$

$25=29+2(ab+ac+bc)$

$2(ab+bc+ac)=-4$

$ab+bc+ac=-2$