Greatest Real Root Comparison
Which of the following polynomials has the greatest real root?
(A) $ x^{19}+2018x^{11}+1 $
(B) $ x^{17}+2018x^{11}+1 $
(C) $ x^{19}+2018x^{13}+1 $
(D) $ x^{17}+2018x^{13}+1 $
(E) $ 2019x+2018 $
Enter the letter of the polynomial with the greatest real root.
- 1
- 2
- 3
- +
- 4
- 5
- 6
- -
- 7
- 8
- 9
- $\frac{a}{b}$
- .
- 0
- =
- %
- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
- $\log$
- $\theta$
- $\sin{}$
- $\cos{}$
- $\tan{}$
- $($
- $)$
- $[$
- $]$
- $\cap$
- $\cup$
- $,$
- $\infty$
Solution
By Descartes' Rule of Signs, none of the polynomials has a positive root, and each one has exactly one negative root. Furthermore, each polynomial is positive at $ x = 0 $ and negative at $ x = -1, $ so each real root lies between $ -1 $ and 0. Also, each polynomial is increasing on the interval $ (-1,0) $.
Let $ r_A $ and $ r_B $ be the roots of the polynomials in options A and B, respectively, so
\[r_A^{19} + 2018r_A^{11} + 1 = r_B^{17} + 2018r_B^{11} + 1 = 0,\]so $ r_A^{19} = r_B^{17} $. Since $ r_A \in (-1,0), $ $ r_B^{17} = r_A^{19} > r_A^{17}, $ so $ r_B > r_A $.
Similarly, let $ r_C $ and $ r_D $ be the roots of the polynomials in options C and D, respectively, so
\[r_C^{19} + 2018r_C^{13} + 1 = r_D^{17} + 2018r_D^{13} + 1 = 0,\]so $ r_C^{19} = r_D^{17} $. Since $ r_C \in (-1,0), $ $ r_D^{17} = r_C^{19} > r_C^{17}, $ so $ r_D > r_C $.
Since
\[r_B^{17} + 2018r_B^{11} + 1 = r_D^{17} + 2018r_D^{13} + 1 = 0,\]we have that $ r_B^{11} = r_D^{13} $. Since $ r_D \in (-1,0), $ $ r_B^{11} = r_D^{13} > r_D^{11}, $ so $ r_B > r_D $.
Therefore, the largest root must be either $ r_B $ or the root of $ 2019x + 2018 = 0, $ which is $ -\frac{2018}{2019} $.
Let $ f(x) = x^{17} + 2018x^{11} + 1, $ so $ f(r_B) = 0 $. Note that
\[f \left( -\frac{2}{3} \right) = -\frac{2^{17}}{3^{17}} - 2018 \cdot \frac{2^{11}}{3^{11}} + 1.\]We claim that $ 2018 \cdot 2^{11} > 3^{11} $. Since $ 2^2 > 3, $ $ 2^{22} > 3^{11} $. Then
\[2018 \cdot 2^{11} = 1009 \cdot 2^{22} > 3^{11}.\]From $ 2018 \cdot 2^{11} > 3^{11}, $ $ 2018 \cdot \frac{2^{11}}{3^{11}} > 1, $ so
\[f \left( -\frac{2}{3} \right) = -\frac{2^{17}}{3^{17}} - 2018 \cdot \frac{2^{11}}{3^{11}} + 1 < 0.\]Since $ f(x) $ is an increasing function, we can conclude that $ r_B > -\frac{2}{3} > -\frac{2018}{2019} $. Therefore, the answer is $ \boxed{\text{(B)}} $.