1. **State the problem:** Find all real zeros of the function $$g(x) = 2x^3 + x^2 - 40x - 20$$ using synthetic division and rational zeros to determine irrational zeros.
2. **Identify possible rational zeros:** By the Rational Root Theorem, possible rational zeros are factors of the constant term $-20$ divided by factors of the leading coefficient $2$. These are $$\pm 1, \pm 2, \pm 4, \pm 5, \pm 10, \pm 20, \pm \frac{1}{2}, \pm \frac{5}{2}$$.
3. **Test rational zeros using synthetic division:** Start with $x=2$.
Synthetic division setup:
$$\begin{array}{r|rrrr}
2 & 2 & 1 & -40 & -20 \\
& & 4 & 10 & -60 \\
\hline
& 2 & 5 & -30 & -80
\end{array}$$
Remainder is $-80 \neq 0$, so $x=2$ is not a zero.
Try $x=-2$:
$$\begin{array}{r|rrrr}
-2 & 2 & 1 & -40 & -20 \\
& & -4 & 6 & 68 \\
\hline
& 2 & -3 & -34 & 48
\end{array}$$
Remainder $48 \neq 0$, so $x=-2$ is not a zero.
Try $x=5$:
$$\begin{array}{r|rrrr}
5 & 2 & 1 & -40 & -20 \\
& & 10 & 55 & 75 \\
\hline
& 2 & 11 & 15 & 55
\end{array}$$
Remainder $55 \neq 0$, so $x=5$ is not a zero.
Try $x=-1$:
$$\begin{array}{r|rrrr}
-1 & 2 & 1 & -40 & -20 \\
& & -2 & 1 & 39 \\
\hline
& 2 & -1 & -39 & 19
\end{array}$$
Remainder $19 \neq 0$, so $x=-1$ is not a zero.
Try $x=\frac{1}{2}$:
$$\begin{array}{r|rrrr}
\frac{1}{2} & 2 & 1 & -40 & -20 \\
& & 1 & 1 & -19.5 \\
\hline
& 2 & 2 & -39 & -39.5
\end{array}$$
Remainder $-39.5 \neq 0$, so $x=\frac{1}{2}$ is not a zero.
Try $x=-5$:
$$\begin{array}{r|rrrr}
-5 & 2 & 1 & -40 & -20 \\
& & -10 & 45 & -25 \\
\hline
& 2 & -9 & 5 & -45
\end{array}$$
Remainder $-45 \neq 0$, so $x=-5$ is not a zero.
Try $x=4$:
$$\begin{array}{r|rrrr}
4 & 2 & 1 & -40 & -20 \\
& & 8 & 36 & -16 \\
\hline
& 2 & 9 & -4 & -36
\end{array}$$
Remainder $-36 \neq 0$, so $x=4$ is not a zero.
Try $x=-\frac{1}{2}$:
$$\begin{array}{r|rrrr}
-\frac{1}{2} & 2 & 1 & -40 & -20 \\
& & -1 & 0 & 20 \\
\hline
& 2 & 0 & -40 & 0
\end{array}$$
Remainder is $0$, so $x = -\frac{1}{2}$ is a zero.
4. **Divide polynomial by $(x + \frac{1}{2})$:** The quotient is $$2x^2 + 0x - 40 = 2x^2 - 40$$.
5. **Solve quadratic $2x^2 - 40 = 0$:**
$$2x^2 - 40 = 0$$
$$2x^2 = 40$$
$$\cancel{2}x^2 = \cancel{2}20$$
$$x^2 = 20$$
$$x = \pm \sqrt{20} = \pm 2\sqrt{5}$$
6. **Final answer:** The real zeros of $g(x)$ are
$$\left\{ -\frac{1}{2}, -2\sqrt{5}, 2\sqrt{5} \right\}$$
These are listed least to greatest in reduced radical and fraction form.
Real Zeros 58Dd0F
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