Hello Guest!
If a,b,c are the solutions of the polynomial equation: \(x^3-4x^2-32x+17=(x-a)(x-b)(x-c)=0\)
Then, by Vietas' formulae: \(a+b+c=4\)
\(ab+ac+bc=-32\)
\(abc=17\)
Using the following identity:
\(y_1^3+y_2^3+y_3^3-3y_1y_2y_3=(y_1+y_2+y_3)(y_1^2+y_2^2+y_3^2-y_1y_2-y_1y_3-y_2y_3)\) (*)
(More commonly written as: \(x^3+y^3+z^3=3xyz\) if and only if \(x+y+z=0\)).
Then:
Let \(y_1=\sqrt[3]{x-a}\) , \(y_2=\sqrt[3]{x-b}\) , \(y_3=\sqrt[3]{x-c}\)
We are given: \(y_1+y_2+y_3=0\)
Hence, (*) becomes:
\((x-a)+(x-b)+(x-c)=3\sqrt[3]{(x-a)(x-b)(x-c)}\)
(Notice: \((x-a)(x-b)(x-c)=x^3-4x^2-32x+17\))
Thus,
\( 3x-(a+b+c) = 3\sqrt[3]{x^3-4x^2-32x+17}\)
(By Vietas formula: a+b+c=4, substitute and cube both sides)
\(\iff (3x-4)^3=27(x^3-4x^2-32x+17)\)
Expanding:
\(\iff 27x^3-108x^2+144x-64=27x^3-108x^2-864x+459\)
Simplify to get a linear equation:
\(144x-64=-864x+459 \implies 1008x=523 \implies x=\frac{523}{1008}\)
Therefore, \(x=\dfrac{523}{1008}\)
.