Question

If a+b+c =6 and a 2 +b 2 +c 2 =20, find ab+bc+ca

Answer 1

(a+b+c)^2=6^2
or a^2+b^2+c^2+2(ab+bc+ca)=36
or 20+2(ab+bc+ca)=36
or 2(ab+bc+ca)=16
or ab+bc+ca=8

Answer 2

Refer to the image below