能使圆x^2+y^2-2x+4y+1=0上恰好。。

能使圆x^2+y^2-2x+4y+1=0上恰好有2个点到直线2x+y+c=0的距离等于1的c值可以为（ )
A.2. B.根号5 C.3 D.3根号5

3(x+y)(x-y)+4(x-y)^2=? 4(x+y)^2+(x+y)+1 2(x-y)-4(-y-x) (2x+y)(2x-y)-(3x-2y)(x+y)-y(2x-y) 1/4(x+y)+1/2(x+y)*(x+y)>=x*根号y+y*根号x 把4（x-y+1)+y(y-2x)因式分解 (x-2y)^2+(x-y)(x-2y)-2(x-3y)(x-y) x^2+3(x+y)+3-y^2+(x-y) y=x*x-2x-2 3X(X-Y)+2Y(X-Y)