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Algebra Difficulty 6.5 National olympiad Find the answer

Find the sum of the real roots of the polynomial k=1100(x211x+k)=(x211x+1)(x211x+2)(x211x+100). \prod_{k=1}^{100} \left( x^2-11x+k \right) = \left( x^2-11x+1 \right)\left( x^2-11x+2 \right)\dots\left(x^2-11x+100\right). [i]Proposed by Evan Chen[/i]

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Solution

To find the sum of the real roots of the polynomial
k=1100(x211x+k)=(x211x+1)(x211x+2)(x211x+100), \prod_{k=1}^{100} \left( x^2-11x+k \right) = \left( x^2-11x+1 \right)\left( x^2-11x+2 \right)\dots\left(x^2-11x+100\right),
we need to analyze the quadratic factors x211x+k x^2 - 11x + k for k=1,2,,100 k = 1, 2, \ldots, 100 .

1. Determine the conditions for real roots:
Each quadratic x211x+k x^2 - 11x + k has real roots if and only if its discriminant is non-negative. The discriminant of x211x+k x^2 - 11x + k is given by:
Δ=b24ac=11241k=1214k. \Delta = b^2 - 4ac = 11^2 - 4 \cdot 1 \cdot k = 121 - 4k.
For the quadratic to have real roots, we need:
1214k0    1214k    k1214=30.25. 121 - 4k \geq 0 \implies 121 \geq 4k \implies k \leq \frac{121}{4} = 30.25.
Since k k must be an integer, the largest possible value of k k is 30. Therefore, the quadratics x211x+k x^2 - 11x + k have real roots for k=1,2,,30 k = 1, 2, \ldots, 30 .

2. Sum of the roots of each quadratic:
By Vieta's formulas, the sum of the roots of the quadratic equation x211x+k=0 x^2 - 11x + k = 0 is given by:
α+β=11. \alpha + \beta = 11.
This is true for each k k from 1 to 30.

3. Total sum of the real roots:
Since there are 30 such quadratics, and each quadratic contributes a sum of 11 for its roots, the total sum of the real roots is:
30×11=330. 30 \times 11 = 330.

The final answer is 330\boxed{330}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.