当x1不等于x2,设直线ab为y=k(x-p/2),和y^2=2px联立解得k²(x-p/2)²=2pxk²x²-k²px-2px+k²p²/4=0x1x2=k²p²/4除以k²=p²/4当x1=x2=p/2,那x1x2=p²/4因为y1,y2一定是一正一负，所以y1y2的绝对值=√2px1*√2px2=√4p²x1x2=√p的4次方=p²那y1y2=-p²
