∵a2+ab-2b2=0
∴（a+2b)(a-b)=0∴a=-2b或者a=b
原式=（a²-b²-a²-b²)/ab
=-2b²/ab
=-2b/a
当a=-2b时,原式=-2b/(-2b)=1；当a=b时,原式=-2b/b=-2