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Printjmc
algebra intermediate
Problem
Which of the following polynomials has the greatest real root?
(A) (B) (C) (D) (E)
Enter the letter of the polynomial with the greatest real root.
(A) (B) (C) (D) (E)
Enter the letter of the polynomial with the greatest real root.
Solution
By Descartes' Rule of Signs, none of the polynomials has a positive root, and each one has exactly one negative root. Furthermore, each polynomial is positive at and negative at so each real root lies between and 0. Also, each polynomial is increasing on the interval
Let and be the roots of the polynomials in options A and B, respectively, so so Since so
Similarly, let and be the roots of the polynomials in options C and D, respectively, so so Since so
Since we have that Since so
Therefore, the largest root must be either or the root of which is
Let so Note that We claim that Since Then From so Since is an increasing function, we can conclude that Therefore, the answer is
Let and be the roots of the polynomials in options A and B, respectively, so so Since so
Similarly, let and be the roots of the polynomials in options C and D, respectively, so so Since so
Since we have that Since so
Therefore, the largest root must be either or the root of which is
Let so Note that We claim that Since Then From so Since is an increasing function, we can conclude that Therefore, the answer is
Final answer
\text{(B)}