a+2b=0
∴a=-2b
把a=-2b代入(a²+2ab-b²)÷(2a²+ab+b²)得
(a²+2ab-b²)÷(2a²+ab+b²)
=(4b²-4b²-b²)÷(8b²-2b²+b²)
=-b²/7b²
=-1/7