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Algebra Difficulty 6.1 National olympiad Prove it Ukraine

It is given that real roots of a quadratic polynomial g(x)=x23x+ag(x) = x^2 - 3x + a are also roots of polynomial f(x)=x3x2+cx+4f(x) = x^3 - x^2 + c x + 4. Analogously, both real roots of a quadratic polynomial h(x)=x2+x+bh(x) = x^2 + x + b are also roots of f(x)f(x). What values can f(1)f(1) take?

Solution

Since f(x)f(x) is a cubic polynomial, it has no more than three real roots, hence, quadratic polynomials g(x)g(x) and h(x)h(x) have the same root. Let us denote this root by tt. Then,
g(t)=t23t+a=0 and h(t)=t2+t+b=04t+ba=0t=14(ab). g(t) = t^2 - 3t + a = 0 \text{ and } h(t) = t^2 + t + b = 0 \Rightarrow 4t + b - a = 0 \Rightarrow t = \frac{1}{4}(a-b).

Then, the equality must hold: f(x)(xt)=g(x)h(x)f(x)(x-t) = g(x)h(x).
(x3x2+cx+4)(xt)=(x23x+a)(x2+x+2). (x^3 - x^2 + c x + 4)(x - t) = (x^2 - 3x + a)(x^2 + x + 2).
By collecting coefficients of x3x^3, we obtain that the equation must be satisfied:
1t=3+1t=1, hence, f(1)=f(t)=0. -1-t = -3+1 \Rightarrow t=1, \text{ hence, } f(1) = f(t) = 0.

It is easy to find an explicit form of polynomials ff, gg and hh, which satisfy the given statement:
f(x)=x3x24x+4,g(x)=x23x+2,h(x)=x2+x2. f(x) = x^3 - x^2 - 4x + 4, \quad g(x) = x^2 - 3x + 2, \quad h(x) = x^2 + x - 2.

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