换元吧
x²+y²=4
令x=2cos𝛉
y=2sin𝛉 𝛉∈(-π,π)
x²+xy-2=4sin²𝛉+4cos𝛉sin𝛉-2
=2(1-cos2𝛉)+2sin2𝛉-2
=2(cos2𝛉+sin2𝛉)=2√2sin(2𝛉+π/4)∈[-2√2,2√2]