Answer:
ab + bc + ca = 40
Step-by-step 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
