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Real Roots of a Quartic

Source: 2018 AIME II #6

March 23, 2018
algebrapolynomialAIMEAIME IAIME II

Problem Statement

A real number aa is chosen randomly and uniformly from the interval [20,18][-20, 18]. The probability that the roots of the polynomial x4+2ax3+(2a2)x2+(4a+3)x2x^4 + 2ax^3 + (2a-2)x^2 + (-4a+3)x - 2 are all real can be written in the form mn\tfrac{m}{n}, where mm and nn are relatively prime positive integers. Find m+nm+n.