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jmc

algebra senior

Problem

Find all real numbers so that has at least two distinct negative real roots.
Solution
We see that cannot be a root of the polynomial. Dividing both sides by we get Let Then so or Hence, If is negative, then by AM-GM, Then Therefore, If then Then so the only negative root is and the condition in the problem is not met. Therefore, and

On the other hand, assume Then by the quadratic formula applied to Since In other words, one of the possible values of is less than

Then from By the quadratic formula, For the value of that is less than both roots are real. Furthermore, their product is 1, so they are both positive or both negative. The sum of the roots is which is negative, so both roots are negative, and since they are distinct.

Therefore, the value of that works are
Final answer
\left( \frac{3}{4}, \infty \right)