焦点F(p/2,0)
设弦所在直线斜率为k,方程为y = k(x - p/2),x = y/k + p/2
y² = 2p(y/k + p/2) = 2py/k + p²
ky² -2py - kp² = 0
y = [p ± p√(k² + 1)]/k
x = [p ± p√(k² + 1)]/k² + p/2
弦长² = 25p²/4 = (x1 - x2)² + (y1 - y2)² = 4p²(k² + 1)²/k⁴
k = ±√6/3
直线的方程y = (±√6/3)(x - p/2)