Question

If a²+b²+c²=20and a+b+c=0 find ac+bc+CA

Answer 1

${(a + b + c)}^{2} \\ = {a}^{2} + {b}^{2} + {c}^{2} + 2ab \: + 2bc + 2ca$
If a+b+c = 0
${0}^{2} = 0$
So 20 + 2(ab +bc +ca) =0
ab+bc+ca= - 10