CPhill

avatar
UsernameCPhill
Score130511
Membership
Stats
Questions 56
Answers 43453

 #1
avatar+130511 
+5

6x^3+38x^2+18x+2=0

This polynomial doesn't factor........here's a graph of the solution.....https://www.desmos.com/calculator/nncnoxpewo

There are three real roots at -5.828, -1/3 and -.172

(We actually could have found the second zero by using the Factor Theorem....this would have led to a quadratic that could be solved with the Quadratic Formula.....!!  )

 

Nov 6, 2014
 #1
avatar+130511 
+5

x^3+2x-5=x+4    rearrange this as

x^3+2x-5-x-4  = 0

x^3+ x - 9  = 0

Using the on-site solver, we have.....

 

$${{\mathtt{x}}}^{{\mathtt{3}}}{\mathtt{\,\small\textbf+\,}}{\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{9}} = {\mathtt{0}} \Rightarrow \left\{ \begin{array}{l}{\mathtt{x}} = {\left({\frac{{\sqrt{{\mathtt{2\,191}}}}}{\left({\mathtt{2}}{\mathtt{\,\times\,}}{{\mathtt{3}}}^{\left({\frac{{\mathtt{3}}}{{\mathtt{2}}}}\right)}\right)}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{9}}}{{\mathtt{2}}}}\right)}^{\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}{\mathtt{\,\times\,}}\left({\mathtt{\,-\,}}{\frac{{\sqrt{{\mathtt{3}}}}{\mathtt{\,\times\,}}{i}}{{\mathtt{2}}}}{\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right){\mathtt{\,-\,}}{\frac{\left({\frac{{\sqrt{{\mathtt{3}}}}{\mathtt{\,\times\,}}{i}}{{\mathtt{2}}}}{\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)}{\left({\mathtt{3}}{\mathtt{\,\times\,}}{\left({\frac{{\sqrt{{\mathtt{2\,191}}}}}{\left({\mathtt{2}}{\mathtt{\,\times\,}}{{\mathtt{3}}}^{\left({\frac{{\mathtt{3}}}{{\mathtt{2}}}}\right)}\right)}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{9}}}{{\mathtt{2}}}}\right)}^{\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}\right)}}\\
{\mathtt{x}} = {\left({\frac{{\sqrt{{\mathtt{2\,191}}}}}{\left({\mathtt{2}}{\mathtt{\,\times\,}}{{\mathtt{3}}}^{\left({\frac{{\mathtt{3}}}{{\mathtt{2}}}}\right)}\right)}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{9}}}{{\mathtt{2}}}}\right)}^{\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}{\mathtt{\,\times\,}}\left({\frac{{\sqrt{{\mathtt{3}}}}{\mathtt{\,\times\,}}{i}}{{\mathtt{2}}}}{\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right){\mathtt{\,-\,}}{\frac{\left({\mathtt{\,-\,}}{\frac{{\sqrt{{\mathtt{3}}}}{\mathtt{\,\times\,}}{i}}{{\mathtt{2}}}}{\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)}{\left({\mathtt{3}}{\mathtt{\,\times\,}}{\left({\frac{{\sqrt{{\mathtt{2\,191}}}}}{\left({\mathtt{2}}{\mathtt{\,\times\,}}{{\mathtt{3}}}^{\left({\frac{{\mathtt{3}}}{{\mathtt{2}}}}\right)}\right)}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{9}}}{{\mathtt{2}}}}\right)}^{\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}\right)}}\\
{\mathtt{x}} = {\left({\frac{{\sqrt{{\mathtt{2\,191}}}}}{\left({\mathtt{2}}{\mathtt{\,\times\,}}{{\mathtt{3}}}^{\left({\frac{{\mathtt{3}}}{{\mathtt{2}}}}\right)}\right)}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{9}}}{{\mathtt{2}}}}\right)}^{\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}{\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{\left({\mathtt{3}}{\mathtt{\,\times\,}}{\left({\frac{{\sqrt{{\mathtt{2\,191}}}}}{\left({\mathtt{2}}{\mathtt{\,\times\,}}{{\mathtt{3}}}^{\left({\frac{{\mathtt{3}}}{{\mathtt{2}}}}\right)}\right)}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{9}}}{{\mathtt{2}}}}\right)}^{\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}\right)}}\\
\end{array} \right\} \Rightarrow \left\{ \begin{array}{l}{\mathtt{x}} = {\mathtt{\,-\,}}{\mathtt{0.960\: \!087\: \!560\: \!673\: \!589\: \!7}}{\mathtt{\,-\,}}{\mathtt{1.940\: \!439\: \!221\: \!537\: \!443\: \!7}}{i}\\
{\mathtt{x}} = {\mathtt{\,-\,}}{\mathtt{0.960\: \!087\: \!560\: \!673\: \!589\: \!7}}{\mathtt{\,\small\textbf+\,}}{\mathtt{1.940\: \!439\: \!221\: \!537\: \!443\: \!7}}{i}\\
{\mathtt{x}} = {\mathtt{1.920\: \!175\: \!121\: \!347\: \!179\: \!5}}\\
\end{array} \right\}$$

 

Thus, there is only one real solution to this.

 

Nov 5, 2014