Question

If a +b+c=9 and a square +b square +square =1 then ab+bc+ca

Answer 1

ab + bc + ca = 40

Explanation:

Using the expansion identity

(a + b + c)² = a² + b² + c² + 2(ab + bc + ca) , that is

9² = 1 + 2(ab + bc + ca)

81 = 1 + 2(ab + bc + ca) ← subtract 1 from both sides

80 = 2(ab + bc + ca) ← divide both sides by 2

40 = ab + bc + ca