×

# 5th degree polynomial

Given the equation $$x^5+5(2x^3+x+2)=5x^2(x^2+2)$$ has a real root in the form of $$a-\sqrt[5]{b}$$ where $$a$$ and $$b$$ are real positive integers. Find $$(a,b)$$

Note by William Isoroku
2 years, 1 month ago

Sort by:

$x^5+5(2x^3+x+2)=5x^2(x^2+2)\\ x^5+10x^3+5x+10 = 5x^4+10x^2 \\ x^5 -5x^4+10x^3-10x^2+5x+10 = 0 \\ x^5 -5x^4+10x^3-10x^2+5x-1+1 +10 = 0 \\ (x-1)^5 = -11 \\ x-1 = - \sqrt [5] {11} \\ x = 1 - \sqrt [5] {11} \\ \Rightarrow \boxed{a = 1 \quad b = 11}$ · 2 years, 1 month ago

Oh, so we have to know the $$(x-1)^5$$ factorization here right? · 2 years, 1 month ago

Yes, it is best that you remember first few rows of the Pascal triangle. They are actual the binomial expansion coefficients, which is given by $$\begin{pmatrix} n \\ r \end{pmatrix}$$. For example, when $$n=5$$, we have:

$$$\begin{split} (x-1)^5 & = \sum_{r=0}^5 {(-1)^r\begin{pmatrix} 5 \\ r \end{pmatrix}x^{5-r}} \\ & = \begin{pmatrix} 5 \\ 0 \end{pmatrix}x^5 - \begin{pmatrix} 5 \\ 1 \end{pmatrix}x^4 + \begin{pmatrix} 5 \\ 2 \end{pmatrix}x^3 - \begin{pmatrix} 5 \\ 3 \end{pmatrix}x^2 + \begin{pmatrix} 5 \\ 4 \end{pmatrix}x^1 - \begin{pmatrix} 5 \\ 5 \end{pmatrix}x^0 \\ & = x^5 - 5x^4 + 10x^3 - 10x^2 + 5x - 1 \end{split}$$$ · 2 years, 1 month ago

How you thought it will be (x-1)^5? Sir. · 2 years, 1 month ago

Pascal triangle:

$1 \\ 1 \quad 1 \\ 1 \quad 2 \quad 1 \\ 1 \quad 3 \quad 3 \quad 1 \\ 1 \quad 4 \quad 6 \quad 4 \quad 1 \\ 1 \quad 5 \quad 10 \quad 10 \quad 5 \quad 1\\ 1 \quad 6 \quad 15 \quad 20 \quad 15 \quad 6 \quad 1$ · 2 years, 1 month ago