Find roots of polynomial $$ p(x) = x^4-21x^2-100x $$

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

$$ \begin{aligned}x_1 &= 0\\[1 em]x_2 &= 6.1123\\[1 em]x_3 &= -3.0562+2.6496i\\[1 em]x_4 &= -3.0562-2.6496i \end{aligned} $$

Step 1:

Factor out $ \color{blue}{ x }$ from $ x^4-21x^2-100x $ and solve two separate equations:

$$ \begin{aligned} x^4-21x^2-100x & = 0\\[1 em] \color{blue}{ x }\cdot ( x^3-21x-100 ) & = 0 \\[1 em] \color{blue}{ x = 0} ~~ \text{or} ~~ x^3-21x-100 & = 0 \end{aligned} $$

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

Step 2:

Polynomial $ x^3-21x-100 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.

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