If a2 + b2 + c2 = 213 and ab + bc + ca = 6, then a + b + c is
Options:  A) 14  B) 20  C) 15  D) 32  E) 64

Rationale: "by formula , ( a + b + c ) ^ 2 = a ^ 2 + b ^ 2 + c ^ 2 + 2 ( ab + bc + ca ) , since , a ^ 2 + b ^ 2 + c ^ 2 = 213 and ab + bc + ca = 6 , ( a + b + c ) ^ 2 = 213 + 2 ( 6 ) = 225 = 15 ^ 2 therefore : a + b + c = 15 answer : c"
Answer: C