Question

Which polynomial equations have -i as one of their roots?

1. x^3 + 5x^2 - 20 = 0

2. x^3 - x^2 + x - 1 = 0

3. x^3 + 3x^2 + x + 3 = 0

4. x^3 - 2x^2 + x - 2 = 0

5. x^3 - 6x^2 - 16x + 96 = 0

Answer 1

i = sqrt(-1)
i^2 = -1
(-i)^2 = (-i)(-i) = i^2 = -1
(-i)^3 = (-i)(-i)(-i) = (-i)(i^2) = i

use these facts to find which equation is correct if x = -i

i bet on 2,3 or 4