1/a-1/b=1/a+b (b-a)/ab=1/(a+b) (b-a)(b+a)/ab=1 (b^2-a^2)/ab=1.(1) then: b/a-a/b-3 =b^2/ab-a^2/ab-3 =(b^2-a^2)/ab-3.把（1）代入 =1-3 =-2
