π/3 或者2π/3设抛物线为y²=2px,设点A(x1,y1)B(x2,y2)则AP=x1+p/2,PB=x2+p/2,∴AB=x1+x2+p交点为(p/2,0)若直线AB不垂直X轴,设直线AB为：y=k(x-p/2),代入抛物线方程,消去y得k²x²-(pk²+2p)x+k
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