y=2x+a代入(2x+a)^2=x4x^2+(4a-1)x+a^2=0x1+x2=-(4a-1)/4y1+y2=(2x1+a)+(2x2+a)=2(x1+x2)+2a=1/2所以中点[(x1+x2)/2,(y1+y2)/2]则x=-(4a-1)/8=-a/2+1/8y=1/4要直线和抛物线有交点必须4x^2+(4a-1)x+a^2=0有解(4a-1)^2...