Answer:
There are four possible combinations of root types for a 7th degree polynomial:
(i) 7 rational roots. ([tex]a_{\mathbb{Q}} = 7[/tex], [tex]a_{\mathbb{I}} = 0[/tex], [tex]a_{\mathbb{C}} = 0[/tex])
(ii) 6 rational roots, 1 irrational root. ([tex]a_{\mathbb{Q}} = 6[/tex], [tex]a_{\mathbb{I}} = 1[/tex], [tex]a_{\mathbb{C}} = 0[/tex])
(iii) 2 rational roots, 1 irrational root, 4 complex roots. ([tex]a_{\mathbb{Q}} = 2[/tex], [tex]a_{\mathbb{I}} = 1[/tex], [tex]a_{\mathbb{C}} = 2[/tex])
(iv) 3 irrational roots, 4 complex roots. ([tex]a_{\mathbb{Q}} = 0[/tex], [tex]a_{\mathbb{I}} = 3[/tex], [tex]a_{\mathbb{C}} = 2[/tex])
Step-by-step explanation:
According to the Fundamental Theorem of Algebra, every polynomial with degree higher than zero has at least a solution and at best [tex]n[/tex] complex roots, where [tex]n[/tex] is the degree of the polynomial. In this case, we have 7 roots.
We can rewrite the 7th degree polynomial as a product of a 3rd degree polynomial and 4th degree polynomial. That is:
[tex]p_{7}(x) = p_{3}(x)\cdot p_{4}(x)[/tex] (Eq. 1)
From Algebra we know that both 3rd degree and 4th degree polynomials are solvable by analytical means:
1) All 4th degree polynomials are solvable by Ferrari's method, whose posible solutions are, respectively:
(i) 4 rational roots.
(ii) 4 irrational roots.
(iii) 2 rational roots, 2 irrational roots.
(iv) 3 rational roots, 1 irrational root.
(v) 1 rational roots, 3 irrational roots.
(vi) 2 rational roots, 2 complex roots.
(vii) 2 irrational roots, 2 complex roots.
(viii) 1 rational root, 1 irrational root, 2 complex roots.
(ix) 4 complex roots.
2) All 3rd degree polynomials are solvable by Cardano's method, whose possible solutions are, respectively:
(i) 3 rational roots.
(ii) 3 irrational roots.
(iii) 1 rational root, 2 complex roots.
(iv) 1 irrational root, 2 complex roots.
(v) 3 complex roots.
Please note that complex roots are always presented in conjugated pairs by Quadratic Formula.
The number of solutions of a polynomials are represented by the following expression:
[tex]n = a_{\mathbb{Q}}+a_{\mathbb{I}}+2\cdot a_{\mathbb{C}}[/tex] (Eq. 2)
Where:
[tex]a_{\mathbb{Q}}[/tex] - Number of rational roots, dimensionless.
[tex]a_{\mathbb{I}}[/tex] - Number of irrational roots, dimensionless.
[tex]a_{\mathbb{C}}[/tex] - Number of pairs of complex roots, dimensionless.
Based on this expression, we find the possible root types for a 7th degree polynomial:
(i) 7 rational roots. ([tex]a_{\mathbb{Q}} = 7[/tex], [tex]a_{\mathbb{I}} = 0[/tex], [tex]a_{\mathbb{C}} = 0[/tex])
(ii) 6 rational roots, 1 irrational root. ([tex]a_{\mathbb{Q}} = 6[/tex], [tex]a_{\mathbb{I}} = 1[/tex], [tex]a_{\mathbb{C}} = 0[/tex])
(iii) 2 rational roots, 1 irrational root, 4 complex roots. ([tex]a_{\mathbb{Q}} = 2[/tex], [tex]a_{\mathbb{I}} = 1[/tex], [tex]a_{\mathbb{C}} = 2[/tex])
(iv) 3 irrational roots, 4 complex roots. ([tex]a_{\mathbb{Q}} = 0[/tex], [tex]a_{\mathbb{I}} = 3[/tex], [tex]a_{\mathbb{C}} = 2[/tex])
