Find roots of polynomial $$ p(x) = 2x^3-11x^2-20x-7 $$

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

$$ \begin{aligned}x_1 &= 7\\[1 em]x_2 &= -1\\[1 em]x_3 &= -\frac{ 1 }{ 2 } \end{aligned} $$

Step 1:

Use rational root test to find out that the $ \color{blue}{ x = 7 } $ is a root of polynomial $ 2x^3-11x^2-20x-7 $.

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}{ 7 } $, with factors of 1 and 7.

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

The POSSIBLE zeroes are:

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

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( 7 \right) = 0 $ so $ x = 7 $ 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-7 }$

$$ \frac{ 2x^3-11x^2-20x-7}{ x-7} = 2x^2+3x+1 $$

Step 2:

The next rational root is $ x = 7 $

$$ \frac{ 2x^3-11x^2-20x-7}{ x-7} = 2x^2+3x+1 $$

Step 3:

The next rational root is $ x = -1 $

$$ \frac{ 2x^2+3x+1}{ x+1} = 2x+1 $$

Step 4:

To find the last zero, solve equation $ 2x+1 = 0 $

$$ \begin{aligned} 2x+1 & = 0 \\[1 em] 2 \cdot x & = -1 \\[1 em] x & = - \frac{ 1 }{ 2 } \end{aligned} $$

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