Find roots of polynomial $$ p(x) = x^3-15x^2-18x $$

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

$$ \begin{aligned}x_1 &= 0\\[1 em]x_2 &= 16.1168\\[1 em]x_3 &= -1.1168 \end{aligned} $$

Step 1:

Factor out $ \color{blue}{ x }$ from $ x^3-15x^2-18x $ and solve two separate equations:

$$ \begin{aligned} x^3-15x^2-18x & = 0\\[1 em] \color{blue}{ x }\cdot ( x^2-15x-18 ) & = 0 \\[1 em] \color{blue}{ x = 0} ~~ \text{or} ~~ x^2-15x-18 & = 0 \end{aligned} $$

One solution is $ \color{blue}{ x = 0 } $. Use second equation to find the remaining roots.

Step 2:

The solutions of $ x^2-15x-18 = 0 $ are: $ x = \dfrac{ 15 }{ 2 }-\dfrac{ 3 \sqrt{ 33}}{ 2 } ~ \text{and} ~ x = \dfrac{ 15 }{ 2 }+\dfrac{ 3 \sqrt{ 33}}{ 2 }$.

You can use step-by-step quadratic equation solver to see a detailed explanation on how to solve this quadratic equation.

Polynomial Roots Calculator