$$-x^5+2x^3+63x = 0$$

$$ \begin{matrix}x_1 = 0 & x_2 = 3 & x_3 = -3 \\[1 em] x_4 = \sqrt{ 7 } i & x_5 = -\sqrt{ 7 } i \\[1 em] \end{matrix} $$

In order to solve $ \color{blue}{ -x^{5}+2x^{3}+63x = 0 } $, first we need to factor our $ x $.

$$ -x^{5}+2x^{3}+63x = x \left( -x^{4}+2x^{2}+63 \right) $$

$ x = 0 $ is a root of multiplicity $ 1 $.

The remaining roots can be found by solving equation $ -x^{4}+2x^{2}+63 = 0$.

$ \color{blue}{ -x^{4}+2x^{2}+63 } $ is a polynomial of degree 4. To find zeros for polynomials of degree 3 or higher we use Rational Root Test.

The Rational Root Theorem tells you that if the polynomial has a rational zero then it must be a fraction $ \dfrac{p}{q} $, where p is a factor of the trailing constant and q is a factor of the leading coefficient.

The factor of the leading coefficient ( -1 ) is 1 .The factors of the constant term (63) are 1 3 7 9 21 63 . Then the Rational Roots Tests yields the following possible solutions:

$$ \pm \frac{ 1 }{ 1 } , ~ \pm \frac{ 3 }{ 1 } , ~ \pm \frac{ 7 }{ 1 } , ~ \pm \frac{ 9 }{ 1 } , ~ \pm \frac{ 21 }{ 1 } , ~ \pm \frac{ 63 }{ 1 } ~ $$

Substitute the POSSIBLE roots one by one into the polynomial to find the actual roots. Start first with the whole numbers.

If we plug these values into the polynomial $ P(x) $, we obtain $ P(3) = 0 $.

To find remaining zeros we use Factor Theorem. This theorem states that if $\frac{p}{q}$ is root of the polynomial then this polynomial can be divided with $ \color{blue}{q x - p} $. In this example:

Divide $ P(x) $ with $ \color{blue}{x - 3} $

$$ \frac{ -x^{4}+2x^{2}+63 }{ \color{blue}{ x - 3 } } = -x^{3}-3x^{2}-7x-21 $$

Polynomial $ -x^{3}-3x^{2}-7x-21 $ can be used to find the remaining roots.

Use the same procedure to find roots of $ -x^{3}-3x^{2}-7x-21 $

When you get second degree polynomial use step-by-step quadratic equation solver to find two remaining roots.

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