Real Numbers Summation
Find the sum of all real numbers $ x $ such that $ 5x^4-10x^3+10x^2-5x-11=0 $.
- 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
Because the problem only asks about the real roots of the polynomial, we can't apply Vieta's formulas directly. Instead, we recognize the coefficients from the expansion of $ (x-1)^5 $: \[(x-1)^5 = x^5 - 5x^4 + 10x^3 - 10x^2 + 5x - 1.\]Seeing this, we subtract $ x^5 $ from both sides, giving \[\begin{aligned} -x^5 + 5x^4 - 10x^3 + 10x^2 - 5x - 11 &= -x^5 \\ -(x-1)^5 - 12 &= -x^5 \\ (x-1)^5 + 12 &= x^5.\end{aligned}\]Hence,
\[x^5 + (1 - x)^5 = 12.\]Let $ x = \frac{1}{2} + y $. Then $ 1 - x = \frac{1}{2} - y, $ so
\[\left( \frac{1}{2} + y \right)^5 + \left( \frac{1}{2} - y \right)^5 = 12.\]This expands to
\[5y^4 + \frac{5}{2} y^2 + \frac{1}{16} = 12.\]Consider the function
\[f(y) = 5y^4 + \frac{5}{2} y^2 + \frac{1}{16}.\]Then $ f(0) = \frac{1}{16}, $ and $ f(y) $ is increasing on $ [0,\infty), $ so there is exactly one positive value of $ y $ for which $ f(y) = 12 $. Also, if $ f(y) = 12, $ then $ f(-y) = 12 $.
This means that there are exactly two solutions in $ x, $ and if $ x $ is one solution, then the other solution is $ 1 - x $. Therefore, the sum of the solutions is $ \boxed{1} $.