(1)∵|? 2 3 |= 2 3 = 8 12 ,|? 3 4 |= 3 4 = 9 12 , 8 12 < 9 12 ,∴? 2 3 >? 3 4 ;(2)∵|-(+2.1)|=2.1,-(-2.1)=2.1,∴|-(+2.1)|=-(-2.1);(3)∵|-3.2|=3.2,|?3 1 8 |=3 1 8 =3.125,3.2>3.125,∴?3.2<?3 1 8 .