x^2+2x+y^2-8y+17=0 x^2+2x+1+y^2-8y+16=0 (x+1)^2+(y-4)^2=0 (x+1)^2≥0,(y-4)^2≥0,(x+1)^2+(y-4)^2≥0 当且仅当(x+1)^2=0,(y-4)^2=0成立时,"="成立 (x+1)^2=0,(y-4)^2=0 x+1=0,y-4=0 x=-1,y=4