8x^2+9y^2+20x-28=0
8x^2+20x+9y^2=28
8(x^2+5x/2)+9y^2=28
8(x^2+5x/2+25/16)+9y^2=28+8*25/16
8(x+5/4)^2+9y^2=81/2
(x+5/4)^2/9+y^2/8=9/16
(x+5/4)^2/(81/16)+y^2/(72/16)=1
所以是椭圆
只不过圆心不在圆心