Divide using synthetic division $$ ( 2x^{4}-3x^{3}-9x^{2}+8x+12 ) \div ( x+1 ) $$
Answer
The synthetic division table is:
$$ \begin{array}{c|rrrrr}-1&2&-3&-9&8&12\\& & -2& 5& 4& \color{black}{-12} \\ \hline &\color{blue}{2}&\color{blue}{-5}&\color{blue}{-4}&\color{blue}{12}&\color{orangered}{0} \end{array} $$The solution is:
$$ \frac{ 2x^{4}-3x^{3}-9x^{2}+8x+12 }{ x+1 } = \color{blue}{2x^{3}-5x^{2}-4x+12} $$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.
$$ \begin{array}{c|rrrrr}\color{blue}{-1}&2&-3&-9&8&12\\& & & & & \\ \hline &&&&& \end{array} $$Step 1 : Bring down the leading coefficient to the bottom row.
$$ \begin{array}{c|rrrrr}-1&\color{orangered}{ 2 }&-3&-9&8&12\\& & & & & \\ \hline &\color{orangered}{2}&&&& \end{array} $$Step 2 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -1 } \cdot \color{blue}{ 2 } = \color{blue}{ -2 } $.
$$ \begin{array}{c|rrrrr}\color{blue}{-1}&2&-3&-9&8&12\\& & \color{blue}{-2} & & & \\ \hline &\color{blue}{2}&&&& \end{array} $$Step 3 : Add down last column: $ \color{orangered}{ -3 } + \color{orangered}{ \left( -2 \right) } = \color{orangered}{ -5 } $
$$ \begin{array}{c|rrrrr}-1&2&\color{orangered}{ -3 }&-9&8&12\\& & \color{orangered}{-2} & & & \\ \hline &2&\color{orangered}{-5}&&& \end{array} $$Step 4 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -1 } \cdot \color{blue}{ \left( -5 \right) } = \color{blue}{ 5 } $.
$$ \begin{array}{c|rrrrr}\color{blue}{-1}&2&-3&-9&8&12\\& & -2& \color{blue}{5} & & \\ \hline &2&\color{blue}{-5}&&& \end{array} $$Step 5 : Add down last column: $ \color{orangered}{ -9 } + \color{orangered}{ 5 } = \color{orangered}{ -4 } $
$$ \begin{array}{c|rrrrr}-1&2&-3&\color{orangered}{ -9 }&8&12\\& & -2& \color{orangered}{5} & & \\ \hline &2&-5&\color{orangered}{-4}&& \end{array} $$Step 6 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -1 } \cdot \color{blue}{ \left( -4 \right) } = \color{blue}{ 4 } $.
$$ \begin{array}{c|rrrrr}\color{blue}{-1}&2&-3&-9&8&12\\& & -2& 5& \color{blue}{4} & \\ \hline &2&-5&\color{blue}{-4}&& \end{array} $$Step 7 : Add down last column: $ \color{orangered}{ 8 } + \color{orangered}{ 4 } = \color{orangered}{ 12 } $
$$ \begin{array}{c|rrrrr}-1&2&-3&-9&\color{orangered}{ 8 }&12\\& & -2& 5& \color{orangered}{4} & \\ \hline &2&-5&-4&\color{orangered}{12}& \end{array} $$Step 8 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -1 } \cdot \color{blue}{ 12 } = \color{blue}{ -12 } $.
$$ \begin{array}{c|rrrrr}\color{blue}{-1}&2&-3&-9&8&12\\& & -2& 5& 4& \color{blue}{-12} \\ \hline &2&-5&-4&\color{blue}{12}& \end{array} $$Step 9 : Add down last column: $ \color{orangered}{ 12 } + \color{orangered}{ \left( -12 \right) } = \color{orangered}{ 0 } $
$$ \begin{array}{c|rrrrr}-1&2&-3&-9&8&\color{orangered}{ 12 }\\& & -2& 5& 4& \color{orangered}{-12} \\ \hline &\color{blue}{2}&\color{blue}{-5}&\color{blue}{-4}&\color{blue}{12}&\color{orangered}{0} \end{array} $$Bottom line represents the quotient $ \color{blue}{ 2x^{3}-5x^{2}-4x+12 } $ with a remainder of $ \color{red}{ 0 } $.