Question

if a+b+c=20 and a^2+b^2+c^2=80 find ab+bc+ca

Answer 1

#cosmic
See the attachment

Answer 2

(a + b + c) = 20
(a² + b² + c²) = 80

we know,
(a + b + c)² = a² + b² + c² + 2(ab + bc + ca)
(20)² = 80 + 2(ab + bc + ca)
400 = 80 + 2(ab + bc + ca)
320/2 = (ab + bc + ca)
160 = (ab + bc + ca )