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 and . If a quadratic polynomial has no real root, then either for all (we write and say is positive) or for all (we write and say is negative).
Lemma. If are (quadratic) polynomials such that the sum of any two of them has no real root and the sum has a real root, then and all cannot have the same sign.
Proof. If and are all positive, then is also positive and thus has no real root, which is a contradiction.
By the lemma, the sums cannot have the same sign. Without loss of generality we can assume that
There are two possibilities.
Case 1: If , then applying the lemma for , we get . This gives the contradiction
Case 2: If , then applying the lemma for , we get . Now applying the lemma for , we get . This gives the contradiction
Hence no such polynomials exist.