若a/b=2,则a^2+b^2/a^2-ab=b^2=?

1.因为a/b=2
所以a=2b
所以(a^2-ab+b^2)/(a^2+b^2)=(4b^2-2b^2+b^2)/(4b^2+b^2)=3/5
2.因为1/a-1/b=3
所以b-a=3ab
所以a-b=-3ab
所以(2a+3ab-2b)/(a-ab-b)=(-6ab+3ab)/(-3ab-ab)=3/4
3.当x=1
等式=1/[1x2]+1/[2x3]+1/[3x4]+……+1/[1999x2000]
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+……+(1/1999-1/2000)
=1-1/2+1/2-1/3+1/3-1/4+……+1/1999-1/2000
=1-1/2000
=1999/2000

题目有误~~