If x + y + z = 6 and x2 + y2 + z2 = 20, then the value of x3 + y3 + z3 - 3xyz is
64
70
72
76
C.
72
x + y + z = 6
S.B.S.
(x + y + z)2 = x2 + y2 + z2 + 2(xy + yz + zx)
⟹ (6)2 = 20 + 2(xy + yz + zx)
⟹ 36 = 20 + 2(xy + yz + zx)
⟹ 36 - 20 = 2(xy + yz + zx)
⟹ 16 = 2(xy + yz + zx)
⟹ (xy + yz + zx) = 8
∴ x3 + y3 + z3 - 3xyz
= (x + y + z) (x2 + y2 + z2 - (xy + yz + zx)
= 6 (20 - 8) = 6 x 12 = 72