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jmc

algebra intermediate

Problem

Let be the three real roots of the equation . Find .
Solution
The expression is not symmetric in the roots so Vieta's formulas can't be used directly to find its value. We hope that we can determine some of the values of the roots explicitly. Letting the equation becomes We can rearrange this as or Therefore, we have It follows that one of the roots of the equation is and the other two roots satisfy the quadratic By Vieta's formulas, the product of the roots of the quadratic is which is negative, so one of the roots must be negative and the other must be positive. Furthermore, the sum of the roots is so the positive root must be greater than Since it follows that is the middle root of the equation. That is,

Then and are the roots of so by Vieta, Thus,
Final answer
2