Q500
If a + b + c = 6, a3 + b3 + c3 - 3abc = 342, then what is the value of ab + bc + ca?
A.
5
B.
8
C.
-7
AnswerD.
-5
Answer: Option C
Solution
Answer: Option C
Solution:
Let c = 0a + b = 6
a3 + b3 = 342
ab = ?
(a3 + b3) = (a + b)[(a + b)2 - 3ab]
342 = 6[62 - 3ab]
57 = 36 - 3ab
3ab = -21
ab = -7