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? 0 -5i -5 5i 5

Answer 1

-5i and 5i cannot be roots of the equation, since they are complex.

Explanation:

A 3-degree polynomial equation must have 3 roots, if one of its roots is a complex number, then its conjugate must also be a root of the function. The problem already stated two roots, which are reals, therefore the last root must also be real. Using this line of thought we know that -5i and 5i cannot be roots of the equation, since they're complex.