Divide using synthetic division $$ ( x^{3}-20x-15 ) \div ( x-3 ) $$
Answer
The synthetic division table is:
$$ \begin{array}{c|rrrr}3&1&0&-20&-15\\& & 3& 9& \color{black}{-33} \\ \hline &\color{blue}{1}&\color{blue}{3}&\color{blue}{-11}&\color{orangered}{-48} \end{array} $$The solution is:
$$ \frac{ x^{3}-20x-15 }{ x-3 } = \color{blue}{x^{2}+3x-11} \color{red}{~-~} \frac{ \color{red}{ 48 } }{ x-3 } $$Explanation
Step 1 : Write down the coefficients of the dividend into division table. Put the zero from $ x -3 = 0 $ ( $ x = \color{blue}{ 3 } $ ) at the left.
$$ \begin{array}{c|rrrr}\color{blue}{3}&1&0&-20&-15\\& & & & \\ \hline &&&& \end{array} $$Step 1 : Bring down the leading coefficient to the bottom row.
$$ \begin{array}{c|rrrr}3&\color{orangered}{ 1 }&0&-20&-15\\& & & & \\ \hline &\color{orangered}{1}&&& \end{array} $$Step 2 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ 3 } \cdot \color{blue}{ 1 } = \color{blue}{ 3 } $.
$$ \begin{array}{c|rrrr}\color{blue}{3}&1&0&-20&-15\\& & \color{blue}{3} & & \\ \hline &\color{blue}{1}&&& \end{array} $$Step 3 : Add down last column: $ \color{orangered}{ 0 } + \color{orangered}{ 3 } = \color{orangered}{ 3 } $
$$ \begin{array}{c|rrrr}3&1&\color{orangered}{ 0 }&-20&-15\\& & \color{orangered}{3} & & \\ \hline &1&\color{orangered}{3}&& \end{array} $$Step 4 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ 3 } \cdot \color{blue}{ 3 } = \color{blue}{ 9 } $.
$$ \begin{array}{c|rrrr}\color{blue}{3}&1&0&-20&-15\\& & 3& \color{blue}{9} & \\ \hline &1&\color{blue}{3}&& \end{array} $$Step 5 : Add down last column: $ \color{orangered}{ -20 } + \color{orangered}{ 9 } = \color{orangered}{ -11 } $
$$ \begin{array}{c|rrrr}3&1&0&\color{orangered}{ -20 }&-15\\& & 3& \color{orangered}{9} & \\ \hline &1&3&\color{orangered}{-11}& \end{array} $$Step 6 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ 3 } \cdot \color{blue}{ \left( -11 \right) } = \color{blue}{ -33 } $.
$$ \begin{array}{c|rrrr}\color{blue}{3}&1&0&-20&-15\\& & 3& 9& \color{blue}{-33} \\ \hline &1&3&\color{blue}{-11}& \end{array} $$Step 7 : Add down last column: $ \color{orangered}{ -15 } + \color{orangered}{ \left( -33 \right) } = \color{orangered}{ -48 } $
$$ \begin{array}{c|rrrr}3&1&0&-20&\color{orangered}{ -15 }\\& & 3& 9& \color{orangered}{-33} \\ \hline &\color{blue}{1}&\color{blue}{3}&\color{blue}{-11}&\color{orangered}{-48} \end{array} $$Bottom line represents the quotient $ \color{blue}{ x^{2}+3x-11 } $ with a remainder of $ \color{red}{ -48 } $.