Find roots of polynomial $$ p(x) = 20x^4+29x^3-497x^2-932x+960 $$

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

$$ \begin{aligned}x_1 &= 5\\[1 em]x_2 &= -4\\[1 em]x_3 &= \frac{ 3 }{ 4 }\\[1 em]x_4 &= -\frac{ 16 }{ 5 } \end{aligned} $$

Step 1:

Use rational root test to find out that the $ \color{blue}{ x = 5 } $ is a root of polynomial $ 20x^4+29x^3-497x^2-932x+960 $.

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}{ 960 } $, with factors of 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 16, 20, 24, 30, 32, 40, 48, 60, 64, 80, 96, 120, 160, 192, 240, 320, 480 and 960.

The leading coefficient is $ \color{red}{ 20 }$, with factors of 1, 2, 4, 5, 10 and 20.

The POSSIBLE zeroes are:

$$ \begin{aligned} \dfrac{\color{blue}{p}}{\color{red}{q}} = & \dfrac{ \text{ factors of 960 }}{\text{ factors of 20 }} = \pm \dfrac{\text{ ( 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 16, 20, 24, 30, 32, 40, 48, 60, 64, 80, 96, 120, 160, 192, 240, 320, 480, 960 ) }}{\text{ ( 1, 2, 4, 5, 10, 20 ) }} = \\[1 em] = & \pm \frac{ 1}{ 1} \pm \frac{ 2}{ 1} \pm \frac{ 3}{ 1} \pm \frac{ 4}{ 1} \pm \frac{ 5}{ 1} \pm \frac{ 6}{ 1} \pm \frac{ 8}{ 1} \pm \frac{ 10}{ 1} \pm \frac{ 12}{ 1} \pm \frac{ 15}{ 1} \pm \frac{ 16}{ 1} \pm \frac{ 20}{ 1} \pm \frac{ 24}{ 1} \pm \frac{ 30}{ 1} \pm \frac{ 32}{ 1} \pm \frac{ 40}{ 1} \pm \frac{ 48}{ 1} \pm \frac{ 60}{ 1} \pm \frac{ 64}{ 1} \pm \frac{ 80}{ 1} \pm \frac{ 96}{ 1} \pm \frac{ 120}{ 1} \pm \frac{ 160}{ 1} \pm \frac{ 192}{ 1} \pm \frac{ 240}{ 1} \pm \frac{ 320}{ 1} \pm \frac{ 480}{ 1} \pm \frac{ 960}{ 1} ~~ \pm \frac{ 1}{ 2} \pm \frac{ 2}{ 2} \pm \frac{ 3}{ 2} \pm \frac{ 4}{ 2} \pm \frac{ 5}{ 2} \pm \frac{ 6}{ 2} \pm \frac{ 8}{ 2} \pm \frac{ 10}{ 2} \pm \frac{ 12}{ 2} \pm \frac{ 15}{ 2} \pm \frac{ 16}{ 2} \pm \frac{ 20}{ 2} \pm \frac{ 24}{ 2} \pm \frac{ 30}{ 2} \pm \frac{ 32}{ 2} \pm \frac{ 40}{ 2} \pm \frac{ 48}{ 2} \pm \frac{ 60}{ 2} \pm \frac{ 64}{ 2} \pm \frac{ 80}{ 2} \pm \frac{ 96}{ 2} \pm \frac{ 120}{ 2} \pm \frac{ 160}{ 2} \pm \frac{ 192}{ 2} \pm \frac{ 240}{ 2} \pm \frac{ 320}{ 2} \pm \frac{ 480}{ 2} \pm \frac{ 960}{ 2} ~~ \pm \frac{ 1}{ 4} \pm \frac{ 2}{ 4} \pm \frac{ 3}{ 4} \pm \frac{ 4}{ 4} \pm \frac{ 5}{ 4} \pm \frac{ 6}{ 4} \pm \frac{ 8}{ 4} \pm \frac{ 10}{ 4} \pm \frac{ 12}{ 4} \pm \frac{ 15}{ 4} \pm \frac{ 16}{ 4} \pm \frac{ 20}{ 4} \pm \frac{ 24}{ 4} \pm \frac{ 30}{ 4} \pm \frac{ 32}{ 4} \pm \frac{ 40}{ 4} \pm \frac{ 48}{ 4} \pm \frac{ 60}{ 4} \pm \frac{ 64}{ 4} \pm \frac{ 80}{ 4} \pm \frac{ 96}{ 4} \pm \frac{ 120}{ 4} \pm \frac{ 160}{ 4} \pm \frac{ 192}{ 4} \pm \frac{ 240}{ 4} \pm \frac{ 320}{ 4} \pm \frac{ 480}{ 4} \pm \frac{ 960}{ 4} ~~ \pm \frac{ 1}{ 5} \pm \frac{ 2}{ 5} \pm \frac{ 3}{ 5} \pm \frac{ 4}{ 5} \pm \frac{ 5}{ 5} \pm \frac{ 6}{ 5} \pm \frac{ 8}{ 5} \pm \frac{ 10}{ 5} \pm \frac{ 12}{ 5} \pm \frac{ 15}{ 5} \pm \frac{ 16}{ 5} \pm \frac{ 20}{ 5} \pm \frac{ 24}{ 5} \pm \frac{ 30}{ 5} \pm \frac{ 32}{ 5} \pm \frac{ 40}{ 5} \pm \frac{ 48}{ 5} \pm \frac{ 60}{ 5} \pm \frac{ 64}{ 5} \pm \frac{ 80}{ 5} \pm \frac{ 96}{ 5} \pm \frac{ 120}{ 5} \pm \frac{ 160}{ 5} \pm \frac{ 192}{ 5} \pm \frac{ 240}{ 5} \pm \frac{ 320}{ 5} \pm \frac{ 480}{ 5} \pm \frac{ 960}{ 5} ~~ \pm \frac{ 1}{ 10} \pm \frac{ 2}{ 10} \pm \frac{ 3}{ 10} \pm \frac{ 4}{ 10} \pm \frac{ 5}{ 10} \pm \frac{ 6}{ 10} \pm \frac{ 8}{ 10} \pm \frac{ 10}{ 10} \pm \frac{ 12}{ 10} \pm \frac{ 15}{ 10} \pm \frac{ 16}{ 10} \pm \frac{ 20}{ 10} \pm \frac{ 24}{ 10} \pm \frac{ 30}{ 10} \pm \frac{ 32}{ 10} \pm \frac{ 40}{ 10} \pm \frac{ 48}{ 10} \pm \frac{ 60}{ 10} \pm \frac{ 64}{ 10} \pm \frac{ 80}{ 10} \pm \frac{ 96}{ 10} \pm \frac{ 120}{ 10} \pm \frac{ 160}{ 10} \pm \frac{ 192}{ 10} \pm \frac{ 240}{ 10} \pm \frac{ 320}{ 10} \pm \frac{ 480}{ 10} \pm \frac{ 960}{ 10} ~~ \pm \frac{ 1}{ 20} \pm \frac{ 2}{ 20} \pm \frac{ 3}{ 20} \pm \frac{ 4}{ 20} \pm \frac{ 5}{ 20} \pm \frac{ 6}{ 20} \pm \frac{ 8}{ 20} \pm \frac{ 10}{ 20} \pm \frac{ 12}{ 20} \pm \frac{ 15}{ 20} \pm \frac{ 16}{ 20} \pm \frac{ 20}{ 20} \pm \frac{ 24}{ 20} \pm \frac{ 30}{ 20} \pm \frac{ 32}{ 20} \pm \frac{ 40}{ 20} \pm \frac{ 48}{ 20} \pm \frac{ 60}{ 20} \pm \frac{ 64}{ 20} \pm \frac{ 80}{ 20} \pm \frac{ 96}{ 20} \pm \frac{ 120}{ 20} \pm \frac{ 160}{ 20} \pm \frac{ 192}{ 20} \pm \frac{ 240}{ 20} \pm \frac{ 320}{ 20} \pm \frac{ 480}{ 20} \pm \frac{ 960}{ 20} ~~ \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( 5 \right) = 0 $ so $ x = 5 $ 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-5 }$

$$ \frac{ 20x^4+29x^3-497x^2-932x+960}{ x-5} = 20x^3+129x^2+148x-192 $$

Step 2:

The next rational root is $ x = 5 $

$$ \frac{ 20x^4+29x^3-497x^2-932x+960}{ x-5} = 20x^3+129x^2+148x-192 $$

Step 3:

The next rational root is $ x = -4 $

$$ \frac{ 20x^3+129x^2+148x-192}{ x+4} = 20x^2+49x-48 $$

Step 4:

The next rational root is $ x = \dfrac{ 3 }{ 4 } $

$$ \frac{ 20x^2+49x-48}{ 4x-3} = 5x+16 $$

Step 5:

To find the last zero, solve equation $ 5x+16 = 0 $

$$ \begin{aligned} 5x+16 & = 0 \\[1 em] 5 \cdot x & = -16 \\[1 em] x & = - \frac{ 16 }{ 5 } \end{aligned} $$

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