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

-10 10 -4 3

-4

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