Divide using synthetic division $$ ( x^{3}-21x-20 ) \div ( x-1 ) $$
Answer
The synthetic division table is:
$$ \begin{array}{c|rrrr}1&1&0&-21&-20\\& & 1& 1& \color{black}{-20} \\ \hline &\color{blue}{1}&\color{blue}{1}&\color{blue}{-20}&\color{orangered}{-40} \end{array} $$The solution is:
$$ \frac{ x^{3}-21x-20 }{ x-1 } = \color{blue}{x^{2}+x-20} \color{red}{~-~} \frac{ \color{red}{ 40 } }{ x-1 } $$Explanation
Step 1 : Write down the coefficients of the dividend into division table. Put the zero from $ x -1 = 0 $ ( $ x = \color{blue}{ 1 } $ ) at the left.
$$ \begin{array}{c|rrrr}\color{blue}{1}&1&0&-21&-20\\& & & & \\ \hline &&&& \end{array} $$Step 1 : Bring down the leading coefficient to the bottom row.
$$ \begin{array}{c|rrrr}1&\color{orangered}{ 1 }&0&-21&-20\\& & & & \\ \hline &\color{orangered}{1}&&& \end{array} $$Step 2 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ 1 } \cdot \color{blue}{ 1 } = \color{blue}{ 1 } $.
$$ \begin{array}{c|rrrr}\color{blue}{1}&1&0&-21&-20\\& & \color{blue}{1} & & \\ \hline &\color{blue}{1}&&& \end{array} $$Step 3 : Add down last column: $ \color{orangered}{ 0 } + \color{orangered}{ 1 } = \color{orangered}{ 1 } $
$$ \begin{array}{c|rrrr}1&1&\color{orangered}{ 0 }&-21&-20\\& & \color{orangered}{1} & & \\ \hline &1&\color{orangered}{1}&& \end{array} $$Step 4 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ 1 } \cdot \color{blue}{ 1 } = \color{blue}{ 1 } $.
$$ \begin{array}{c|rrrr}\color{blue}{1}&1&0&-21&-20\\& & 1& \color{blue}{1} & \\ \hline &1&\color{blue}{1}&& \end{array} $$Step 5 : Add down last column: $ \color{orangered}{ -21 } + \color{orangered}{ 1 } = \color{orangered}{ -20 } $
$$ \begin{array}{c|rrrr}1&1&0&\color{orangered}{ -21 }&-20\\& & 1& \color{orangered}{1} & \\ \hline &1&1&\color{orangered}{-20}& \end{array} $$Step 6 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ 1 } \cdot \color{blue}{ \left( -20 \right) } = \color{blue}{ -20 } $.
$$ \begin{array}{c|rrrr}\color{blue}{1}&1&0&-21&-20\\& & 1& 1& \color{blue}{-20} \\ \hline &1&1&\color{blue}{-20}& \end{array} $$Step 7 : Add down last column: $ \color{orangered}{ -20 } + \color{orangered}{ \left( -20 \right) } = \color{orangered}{ -40 } $
$$ \begin{array}{c|rrrr}1&1&0&-21&\color{orangered}{ -20 }\\& & 1& 1& \color{orangered}{-20} \\ \hline &\color{blue}{1}&\color{blue}{1}&\color{blue}{-20}&\color{orangered}{-40} \end{array} $$Bottom line represents the quotient $ \color{blue}{ x^{2}+x-20 } $ with a remainder of $ \color{red}{ -40 } $.