5.5 Zeros of polynomial functions  (Page 3/14)

The only possible rational zeros of $f(x)$ are the quotients of the factors of the last term, –4, and the factors of the leading coefficient, 2.

The constant term is –4; the factors of –4 are $p=\mathrm{\pm 1},\mathrm{\pm 2},\mathrm{\pm 4.}$

The leading coefficient is 2; the factors of 2 are $q=\mathrm{\pm 1},\mathrm{\pm 2.}$

If any of the four real zeros are rational zeros, then they will be of one of the following factors of –4 divided by one of the factors of 2.

Note that $\frac{2}{2}=1$ and $\frac{4}{2}=2,$ which have already been listed. So we can shorten our list.

The Rational Zero Theorem tells us that if $\frac{p}{q}$ is a zero of $f(x),$ then $p$ is a factor of 1 and $q$ is a factor of 2.

The factors of 1 are $\mathrm{\pm 1}$ and the factors of 2 are $\mathrm{\pm 1}$ and $\mathrm{\pm 2.}$ The possible values for $\frac{p}{q}$ are $\mathrm{\pm 1}$ and $\pm \frac{1}{2}.$ These are the possible rational zeros for the function. We can determine which of the possible zeros are actual zeros by substituting these values for $x$ in $f(x).$

Of those, $\mathrm{-1,}-\frac{1}{2},\text{ and }\frac{1}{2}$ are not zeros of $f(x).$ 1 is the only rational zero of $f(x).$

The Rational Zero Theorem tells us that if $\frac{p}{q}$ is a zero of $f(x),$ then $p$ is a factor of –1 and $q$ is a factor of 4.

The factors of $\mathrm{–1}$ are $\mathrm{\pm 1}$ and the factors of $4$ are $\mathrm{\pm 1},\mathrm{\pm 2},$ and $\mathrm{\pm 4.}$ The possible values for $\frac{p}{q}$ are $\mathrm{\pm 1},\pm \frac{1}{2},$ and $\pm \frac{1}{4}.$ These are the possible rational zeros for the function. We will use synthetic division to evaluate each possible zero until we find one that gives a remainder of 0. Let’s begin with 1.

Dividing by $(x-1)$ gives a remainder of 0, so 1 is a zero of the function. The polynomial can be written as

The quadratic is a perfect square. $f(x)$ can be written as

We already know that 1 is a zero. The other zero will have a multiplicity of 2 because the factor is squared. To find the other zero, we can set the factor equal to 0.

The zeros of the function are 1 and $-\frac{1}{2}$ with multiplicity 2.