当－1< a < 0 时,可假设a = -1/2,
则1/a = -2 ,故 a > 1/a
当 a= -1 时,1/a = -1 ; 故a = 1/a
当a < -1 时,可假设a = -2 ,
则 1/a = -1/2 ,故 a < 1/a