If a2 + b2 + c2 = 560 and ab + bc + ca =8, then a + b + c is Pick one:
A) 4 B) 52 C) 16 D) 24 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 = 560 and ab + bc + ca = 8 , ( a + b + c ) ^ 2 = 560 + 2 ( 8 ) = 576 = 24 ^ 2 therefore : a + b + c = 24 answer : d"
Answer: D