Question

Two roots of a 3-degree polynomial equation are 5 and -5. Which of the following cannot be the third root of the equation?

5

5i

0

-5

-5i

Answer 1

Your answer would be 0. Then it would be a 2-degree.

Answer 2

5i, -5i

Explanation:

None 5i nor -5i could be a third root of the polynomial because if we had a complex root then by the complex conjugate root theorem, their conjugate would be a fourth root, while the polynomial which is a 3-degree polynomial can only have three roots according to the Fundamental Theorem of Algebra, which states that the number of roots of the polynomial is equal to the degree of the polynomial.

In other words, for example, if the third root was 5i then its conjugate -5i should be another root according to the complex conjugate root theorem; so we would end with four roots when we are only allowed -by the Fundamental Theorem of Algebra- to have three roots since it is a 3-degree polynomial. The same reasoning applies for -5i whose conjugate is precisely 5i.

The other numbers are real numbers, so any of them can be a third root. Notice that it does not matter if the third root is again 5 or -5 since the real roots can be repeated (multiplicity greater than 1).