Find roots of polynomial $$ p(x) = 15x^4-4x $$
Answer
The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0\\[1 em]x_2 &= 0.6437\\[1 em]x_3 &= -0.3218+0.5574i\\[1 em]x_4 &= -0.3218-0.5574i \end{aligned} $$Explanation
Step 1:
Factor out $ \color{blue}{ x }$ from $ 15x^4-4x $ and solve two separate equations:
$$ \begin{aligned} 15x^4-4x & = 0\\[1 em] \color{blue}{ x }\cdot ( 15x^3-4 ) & = 0 \\[1 em] \color{blue}{ x = 0} ~~ \text{or} ~~ 15x^3-4 & = 0 \end{aligned} $$One solution is $ \color{blue}{ x = 0 } $. Use second equation to find the remaining roots.
Step 2:
Polynomial $ 15x^3-4 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.
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