Find roots of polynomial $$ p(x) = x^3+21x $$

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

$$ \begin{aligned}x_1 &= 0\\[1 em]x_2 &= \sqrt{ 21 }i\\[1 em]x_3 &= -\sqrt{ 21 }i \end{aligned} $$

Step 1:

Factor out $ \color{blue}{ x }$ from $ x^3+21x $ and solve two separate equations:

$$ \begin{aligned} x^3+21x & = 0\\[1 em] \color{blue}{ x }\cdot ( x^2+21 ) & = 0 \\[1 em] \color{blue}{ x = 0} ~~ \text{or} ~~ x^2+21 & = 0 \end{aligned} $$

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

Step 2:

The solutions of $ x^2+21 = 0 $ are: $ x = \sqrt{ 21 } i ~ \text{and} ~ x = -\sqrt{ 21 } i $.

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

Polynomial Roots Calculator