Question

Write a polynomial function in standard form given the zeros.

-1/2, -5 + i

Answer 1

y = x³ + 10.5x² + 31x + 13

Explanation:

Complex roots (roots that have imaginary terms) always come in conjugate pairs. So if one root is -5 + i, there's another root that's -5 − i.

So the polynomial is:

y = (x + 1/2) (x − (-5 + i)) (x − (-5 − i))

Distributing:

y = (x + 1/2) (x² − (-5 + i)x − (-5 − i)x + (-5 + i)(-5 − i))

y = (x + 1/2) (x² + 5x − ix + 5x + ix + (-5 + i)(-5 − i))

y = (x + 1/2) (x² + 10x + (-5 + i)(-5 − i))

y = (x + 1/2) (x² + 10x + 25 + 5i − 5i − i²)

y = (x + 1/2) (x² + 10x + 25 + 1)

y = (x + 1/2) (x² + 10x + 26)

y = x(x² + 10x + 26) + 1/2(x² + 10x + 26)

y = x³ + 10x² + 26x + 1/2x² + 5x + 13

y = x³ + 10.5x² + 31x + 13