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Algebra Difficulty 6.5 National olympiad Find the answer

What is the sum of the real roots of the equation 4x43x2+7x3=04x^4-3x^2+7x-3=0?

Pick one

Solution

1. Given the polynomial equation:
4x43x2+7x3=0 4x^4 - 3x^2 + 7x - 3 = 0
we need to find the sum of the real roots.

2. We start by using synthetic division to check for possible rational roots. Let's test x=12 x = \frac{1}{2} :
Synthetic division of 4x43x2+7x3 by (2x1): \text{Synthetic division of } 4x^4 - 3x^2 + 7x - 3 \text{ by } (2x - 1):
2x14037342160 \begin{array}{r|rrrrr} 2x - 1 & 4 & 0 & -3 & 7 & -3 \\ \hline & 4 & 2 & -1 & 6 & 0 \\ \end{array}
The remainder is 0, so x=12 x = \frac{1}{2} is a root, and 4x43x2+7x3 4x^4 - 3x^2 + 7x - 3 can be factored as:
4x43x2+7x3=(2x1)(2x3+x2x+3) 4x^4 - 3x^2 + 7x - 3 = (2x - 1)(2x^3 + x^2 - x + 3)

3. Next, we apply synthetic division again to the cubic polynomial 2x3+x2x+3 2x^3 + x^2 - x + 3 to check for another rational root. Let's test x=32 x = -\frac{3}{2} :
Synthetic division of 2x3+x2x+3 by (2x+3): \text{Synthetic division of } 2x^3 + x^2 - x + 3 \text{ by } (2x + 3):
2x+321132220 \begin{array}{r|rrrr} 2x + 3 & 2 & 1 & -1 & 3 \\ \hline & 2 & -2 & 2 & 0 \\ \end{array}
The remainder is 0, so x=32 x = -\frac{3}{2} is a root, and 2x3+x2x+3 2x^3 + x^2 - x + 3 can be factored as:
2x3+x2x+3=(2x+3)(x2x+1) 2x^3 + x^2 - x + 3 = (2x + 3)(x^2 - x + 1)

4. Now, we have:
4x43x2+7x3=(2x1)(2x+3)(x2x+1) 4x^4 - 3x^2 + 7x - 3 = (2x - 1)(2x + 3)(x^2 - x + 1)

5. The quadratic x2x+1 x^2 - x + 1 has no real roots because its discriminant is negative:
Δ=(1)2411=14=3 \Delta = (-1)^2 - 4 \cdot 1 \cdot 1 = 1 - 4 = -3

6. Therefore, the only real roots of the original polynomial are x=12 x = \frac{1}{2} and x=32 x = -\frac{3}{2} .

7. The sum of the real roots is:
12+(32)=1232=1 \frac{1}{2} + \left(-\frac{3}{2}\right) = \frac{1}{2} - \frac{3}{2} = -1

The final answer is 1\boxed{-1}.

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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.