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Algebra Difficulty 6.0 AIME, harder Prove it Mongolia

Do there exist four quadratic polynomials such that the sum of any three of them has a real root, but the sum of any two of them has no real root?

Solution

Answer: no.
Assume that there exist such four quadratic polynomials f1,f2,f3f_1, f_2, f_3 and f4f_4. If a quadratic polynomial f(x)f(x) has no real root, then either f(x)>0f(x) > 0 for all xRx \in \mathbb{R} (we write f>0f > 0 and say ff is positive) or f(x)<0f(x) < 0 for all xRx \in \mathbb{R} (we write f<0f < 0 and say ff is negative).

Lemma. If g1,g2,g3g_1, g_2, g_3 are (quadratic) polynomials such that the sum of any two of them has no real root and the sum g1+g2+g3g_1 + g_2 + g_3 has a real root, then g1+g2,g1+g3g_1 + g_2, g_1 + g_3 and g2+g3g_2 + g_3 all cannot have the same sign.

Proof. If g1+g2,g1+g3g_1 + g_2, g_1 + g_3 and g2+g3g_2 + g_3 are all positive, then g1+g2+g3g_1 + g_2 + g_3 is also positive and thus has no real root, which is a contradiction. \square

By the lemma, the sums f1+f2,f1+f3,f2+f3f_1 + f_2, f_1 + f_3, f_2 + f_3 cannot have the same sign. Without loss of generality we can assume that
f1+f2>0f1+f3>0f2+f3<0. \begin{aligned} f_1 + f_2 &> 0 \\ f_1 + f_3 &> 0 \\ f_2 + f_3 &< 0. \end{aligned}

There are two possibilities.

Case 1: If f2+f4>0f_2 + f_4 > 0, then applying the lemma for f1,f2,f4f_1, f_2, f_4, we get f1+f4<0f_1 + f_4 < 0. This gives the contradiction
0>(f2+f3)+(f1+f4)=(f1+f3)+(f2+f4)>0. 0 > (f_2 + f_3) + (f_1 + f_4) = (f_1 + f_3) + (f_2 + f_4) > 0.

Case 2: If f2+f4<0f_2 + f_4 < 0, then applying the lemma for f2,f3,f4f_2, f_3, f_4, we get f3+f4>0f_3 + f_4 > 0. Now applying the lemma for f1,f3,f4f_1, f_3, f_4, we get f1+f4<0f_1 + f_4 < 0. This gives the contradiction
0>(f2+f3)+(f1+f4)=(f1+f2)+(f3+f4)>0. 0 > (f_2 + f_3) + (f_1 + f_4) = (f_1 + f_2) + (f_3 + f_4) > 0.
Hence no such polynomials exist.

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