Question

If a?+b?+c?=80 a+b+c=20 Find ab+bc+ca

Answer 1

a2+b2+c2=(a+b+c)2-2(ab+bc+ca)
⇒ab+bc+ca= (a+b+c)2-(a2+b2+c2)/2=20^2-80/2=400-80/2=320/2=160