Divide using synthetic division $$ ( x^{3}+9x^{2}-12x-20 ) \div ( x+10 ) $$
Answer
The synthetic division table is:
$$ \begin{array}{c|rrrr}-10&1&9&-12&-20\\& & -10& 10& \color{black}{20} \\ \hline &\color{blue}{1}&\color{blue}{-1}&\color{blue}{-2}&\color{orangered}{0} \end{array} $$The solution is:
$$ \frac{ x^{3}+9x^{2}-12x-20 }{ x+10 } = \color{blue}{x^{2}-x-2} $$Explanation
Step 1 : Write down the coefficients of the dividend into division table. Put the zero from $ x + 10 = 0 $ ( $ x = \color{blue}{ -10 } $ ) at the left.
$$ \begin{array}{c|rrrr}\color{blue}{-10}&1&9&-12&-20\\& & & & \\ \hline &&&& \end{array} $$Step 1 : Bring down the leading coefficient to the bottom row.
$$ \begin{array}{c|rrrr}-10&\color{orangered}{ 1 }&9&-12&-20\\& & & & \\ \hline &\color{orangered}{1}&&& \end{array} $$Step 2 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -10 } \cdot \color{blue}{ 1 } = \color{blue}{ -10 } $.
$$ \begin{array}{c|rrrr}\color{blue}{-10}&1&9&-12&-20\\& & \color{blue}{-10} & & \\ \hline &\color{blue}{1}&&& \end{array} $$Step 3 : Add down last column: $ \color{orangered}{ 9 } + \color{orangered}{ \left( -10 \right) } = \color{orangered}{ -1 } $
$$ \begin{array}{c|rrrr}-10&1&\color{orangered}{ 9 }&-12&-20\\& & \color{orangered}{-10} & & \\ \hline &1&\color{orangered}{-1}&& \end{array} $$Step 4 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -10 } \cdot \color{blue}{ \left( -1 \right) } = \color{blue}{ 10 } $.
$$ \begin{array}{c|rrrr}\color{blue}{-10}&1&9&-12&-20\\& & -10& \color{blue}{10} & \\ \hline &1&\color{blue}{-1}&& \end{array} $$Step 5 : Add down last column: $ \color{orangered}{ -12 } + \color{orangered}{ 10 } = \color{orangered}{ -2 } $
$$ \begin{array}{c|rrrr}-10&1&9&\color{orangered}{ -12 }&-20\\& & -10& \color{orangered}{10} & \\ \hline &1&-1&\color{orangered}{-2}& \end{array} $$Step 6 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -10 } \cdot \color{blue}{ \left( -2 \right) } = \color{blue}{ 20 } $.
$$ \begin{array}{c|rrrr}\color{blue}{-10}&1&9&-12&-20\\& & -10& 10& \color{blue}{20} \\ \hline &1&-1&\color{blue}{-2}& \end{array} $$Step 7 : Add down last column: $ \color{orangered}{ -20 } + \color{orangered}{ 20 } = \color{orangered}{ 0 } $
$$ \begin{array}{c|rrrr}-10&1&9&-12&\color{orangered}{ -20 }\\& & -10& 10& \color{orangered}{20} \\ \hline &\color{blue}{1}&\color{blue}{-1}&\color{blue}{-2}&\color{orangered}{0} \end{array} $$Bottom line represents the quotient $ \color{blue}{ x^{2}-x-2 } $ with a remainder of $ \color{red}{ 0 } $.