a^2+ab-2b^2=0
(a-b)(a+2b)=0
a=b或a=-2b
a=b时,
(2a-b)/(2a+b)
=(2b-b)/(2b+b)
=b/3b
=1/3
a=-2b时,
(2a-b)/(2a+b)
=(-4b-b)/(-4b+b)
=(-5b)/(-3b)
=5/3