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Four solutions form an arithmetic progression

Source: Vietnam NMO 1985 Problem 5

February 12, 2009
parameterizationalgebra unsolvedalgebra

Problem Statement

Find all real values of parameter a a for which the equation in x x 16x^4 \minus{} ax^3 \plus{} (2a \plus{} 17)x^2 \minus{} ax \plus{} 16 \equal{} 0 has four solutions which form an arithmetic progression.