1. If a polynomial function with rational coefficients has -4 and 8 as roots, it means we can factor this polynomial this way :
(x - (-4))(x-8) = (x+4)(x-8) = x² - 8x + 4x - 32 = x² - 4x - 32
1x² - 4x - 32 is a polynomial function of least degree with rational coefficients that satisifies the given conditions. The leading coefficient here is 1, we call this a monic polynomial.
2. There may be something missing in the question here
3. We can use the factoring techniques to find the roots :
x^3 - 5x² + 9x - 45 let us group x^3 with -5x² and 9x with -45 :
(x^3 - 5x²) + (9x - 45) = x²(x - 5) + 9(x - 5)
We can notice now that there is a common factor : (x-5) this allows us to factor again by (x-5) :
Y = (x-5)(x²+9)
This is the maximum we can do on R, but on C the set of complex we can still factor again.
x² + 9 has a negative discriminant : Δ = b² - 4ac where b = 0 , a = 1, c = 9
Δ < 0, Δ = - 36
Applying the formulas for the roots x1 and x2 we get x1 = -3i, and x2 = + 3i
Y = (x-5)(x-3i)(x+3i)
So the solution is S = {5, 3i, -3i} the three roots of the function.
