原式=2[x^2+1/2xy+(y/4)^2]-y^2-1/8*y^2
=2(x+y/4)^2-9/8*y^2
=[√2(x+y/4)+(√9/8)y]*[√2(x+y/4)-(√9/8)y]
上式再化减一下就行了.