Factor polynomial $$ x^{3}+4x^{2}-25x-100 $$

$$ x^{3}+4x^{2}-25x-100 = ( x-5 )( x+5 )( x+4 ) $$

Step 1 :

To factor $ x^{3}+4x^{2}-25x-100 $ we can use factoring by grouping:

Group $ \color{blue}{ x^{3} }$ with $ \color{blue}{ 4x^{2} }$ and $ \color{red}{ -25x }$ with $ \color{red}{ -100 }$ then factor each group.

$$ \begin{aligned} x^{3}+4x^{2}-25x-100 = ( \color{blue}{ x^{3}+4x^{2} } ) + ( \color{red}{ -25x-100 }) &= \\ &= \color{blue}{ x^{2}( x+4 )} + \color{red}{ -25( x+4 ) } = \\ &= (x^{2}-25)(x+4) \end{aligned} $$

Step 2 :

Rewrite $ x^{2}-25 $ as:

$$ x^{2}-25 = (x)^2 - (5)^2 $$

Now we can apply the difference of squares formula.

$$ I^2 - II^2 = (I - II)(I + II) $$

After putting $ I = x $ and $ II = 5 $ , we have:

$$ x^{2}-25 = (x)^2 - (5)^2 = ( x-5 ) ( x+5 ) $$

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