Find roots of polynomial $$ p(x) = -2x^4-x^3+189 $$

The roots of polynomial $ p(x) $ are:

$$ \begin{aligned}x_1 &= 3\\[1 em]x_2 &= -3.2508\\[1 em]x_3 &= -0.1246+3.1104i\\[1 em]x_4 &= -0.1246-3.1104i \end{aligned} $$

Step 1:

Use rational root test to find out that the $ \color{blue}{ x = 3 } $ is a root of polynomial $ -2x^4-x^3+189 $.

The Rational Root Theorem tells us that if the polynomial has a rational zero then it must be a fraction $ \dfrac{ \color{blue}{p}}{ \color{red}{q} } $, where $ p $ is a factor of the constant term and $ q $ is a factor of the leading coefficient.

The constant term is $ \color{blue}{ 189 } $, with factors of 1, 3, 7, 9, 21, 27, 63 and 189.

The leading coefficient is $ \color{red}{ 2 }$, with factors of 1 and 2.

The POSSIBLE zeroes are:

$$ \begin{aligned} \dfrac{\color{blue}{p}}{\color{red}{q}} = & \dfrac{ \text{ factors of 189 }}{\text{ factors of 2 }} = \pm \dfrac{\text{ ( 1, 3, 7, 9, 21, 27, 63, 189 ) }}{\text{ ( 1, 2 ) }} = \\[1 em] = & \pm \frac{ 1}{ 1} \pm \frac{ 3}{ 1} \pm \frac{ 7}{ 1} \pm \frac{ 9}{ 1} \pm \frac{ 21}{ 1} \pm \frac{ 27}{ 1} \pm \frac{ 63}{ 1} \pm \frac{ 189}{ 1} ~~ \pm \frac{ 1}{ 2} \pm \frac{ 3}{ 2} \pm \frac{ 7}{ 2} \pm \frac{ 9}{ 2} \pm \frac{ 21}{ 2} \pm \frac{ 27}{ 2} \pm \frac{ 63}{ 2} \pm \frac{ 189}{ 2} ~~ \end{aligned} $$

Substitute the possible roots one by one into the polynomial to find the actual roots. Start first with the whole numbers.

We can see that $ p\left( 3 \right) = 0 $ so $ x = 3 $ is a root of a polynomial $ p(x) $.

To find remaining zeros we use Factor Theorem. This theorem states that if $ \dfrac{p}{q} $ is root of the polynomial then the polynomial can be divided by $ \color{blue}{qx − p} $. In this example we divide polynomial $ p $ by $ \color{blue}{ x-3 }$

$$ \frac{ -2x^4-x^3+189}{ x-3} = -2x^3-7x^2-21x-63 $$

Step 2:

The next rational root is $ x = 3 $

$$ \frac{ -2x^4-x^3+189}{ x-3} = -2x^3-7x^2-21x-63 $$

Step 3:

Polynomial $ -2x^3-7x^2-21x-63 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.

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