y^2=4x=2px,p=2,故准线方程是x=-p/2=-1,即N(-1,0)
A(X1,Y1),B(X2,Y2)
向量AB=(x2-x1,y2-y1),NA=(x1+1,y1)
AB=2NA
即有y2-y1=2y1,x2-x1=2x1+2
y2=3y1
x2=3x1+2
y2^2=4x2
y1^2=4x1
y2^2/y1^2=x2/x1=9
x2=9x1
9x1=3x1+2
x1=1/3, y1=2/3根号3或-2/3根号3
x2=3, y2=2根号3或-2根号3
K=(y2-y1)/(x2-x1)=(2根号3-2/3根号3)/(3-1/3)=根号3/2
或者说=(-2根号3+2/3根号3)/(3-1/3)=-根号3/2