IMO 1985 LL USA88

Determine the range of w(w + x)(w + y)(w + z), where x, y,

IMO 1985 LL USA88

Origin: USA

Problem

Determine the range of w(w + x)(w + y)(w + z), where x, y, z, and w are real numbers such that x + y + z + w = x7 + y7 + z7 + w7 = 0.