I must find 3rd degree Polynomial functions in R[x] with:

1) no roots

2) only one root

3) only two roots

4) only 3 roots

If the function has a root, then prove it. If not, then explain why.

My attempt:

We know, that the cubic function can have one, two or three roots. But I really don't know, how I can find the polynomial functions.

1) Explanation

A 3rd polynomial function can not have no root because a polynomial function have at least one root. (Continous function)

  • 1
    $\begingroup$ By root, do you mean only real roots, such as $-1, \pi$ etc.?All polynomials of degree $n$ in one variable have exactly $n$ complex roots, counting multiplicity. $\endgroup$ Commented Dec 6, 2018 at 15:43
  • 1
    $\begingroup$ What about $(x - a)^3$ and $(x-a)^2(x-b)$ and $(x-a)(x-b)(x-c)$ where $a, \, b,\,c$ are distinct numbers, where "numbers" is whatever type of numbers you need this for (rational numbers, real numbers, complex numbers, positive integers with exactly five prime divisors, etc.)? $\endgroup$ Commented Dec 6, 2018 at 15:57
  • 1
    $\begingroup$ For part 1, the continuity is not enough. Are you saying that there is no continuous function that has no roots? You also need to say that the function changes sign, since the limits at $+\infty$ and $-\infty$ have opposite sign. Since it's continuous, it means that it reaches all values between the limits, including $0$. $\endgroup$
    – Andrei
    Commented Dec 6, 2018 at 16:05
  • $\begingroup$ Its important to clarify if you are counting multiplicity or not. No degree three polynomial has two real roots if you count multiplicity, but there are degree three polynomials with only two distinct real roots. $\endgroup$
    – Eric
    Commented Dec 6, 2018 at 16:47

1 Answer 1


Hint: make them in a standard form as $(x-x_1)(x-x_2)(x-x_3)$ where $x_1,x_2,x_3$ are the roots. For example for the case where we have $2$ zero roots we have $$x_1=x_2=0$$and choosing $x_3=1$ arbitrarily we obtain$$f(x)=x^2(x-1)$$


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .