x^2/5+6xy/5+9y^2/5
=1/5(x^2+6xy+y^2)
=1/5(x+3y)^2
-x^2-1/9+2x/3
=-(x^2-2x/3+1/9)
=-(x-1/3)^2^是什么 能用我的表达形式么