If x2 + x + 1 = 0, then the value of (x + 1x )4 + (x2 + 1x2 )4 + (x3 + 1x3 )4 + + (x25 + 1x25 )4 is:
- 162
- 145
- 128
- 175
Answer
(2) 145
Solution
x2 + x + 1 = 0 x = or 2, where 3 = 1 and 1 + + 2 = 0.
xn + 1xn = n + 2n
If n is a multiple of 3: n + 2n = 1 + 1 = 2 term = 24 = 16
If n is not a multiple of 3: n + 2n = + 2 = -1 term = (-1)4 = 1
For n = 1 to 25, multiples of 3 are 3, 6, …, 24 8 terms; remaining 17 terms.
Sum = 8 16 + 17 1 = 128 + 17 = 145
Answer: (2)