X1+x2=a x1*x2=a-2
︳x1-x2︳ ＝√（x1+x2）2-4x1x2=√a2-4(a-2)= √a2-4a+8
=√a2-4a+4+4=√(a-2)2+4
当a=2时 (a-2)2＝0 ︳x1-x2︳取最小值
︳x1-x2 ︳最小值＝2