x^2 + 3x + 5 = 7x^2 + 3x = 2x^2 + 3x + 9/4 = 2 + 9/4( x + 3/2)^2 = 17 / 4此时又分两种情况:一、x + 3 / 2 = 根号下17/4x = 根号下17/4 - 3/2二、x + 3/2 = - 根号下17/4x = - 根号下17/4 - 3/2...