If a + b + c = 5 a3 + b3 +c3 - 3abc = 185, then Find ab + bc + ca

Put c = 0 as there are three variables and two equations.

a + b = 5

a³ + b³ = 185, find ab

⇒(a + b)³ =  a³ + b³ + 3ab(a + b)

⇒ (5)³ = a³ + b³ + 3ab(5)

⇒ 125 = 185 + 15ab

⇒ 15ab = -60

ab = -4