x^2 = 2py
y = x^2/(2p)
y' = x/p
设A(a, a^2/(2p)), B(b, b^2/(2p))
直线l 的方程: [y - b^2/(2p)]/(x - b) = [a^2/(2p) - b^2/(2p)]/(b -a)
直线l 过P(0,p), 代入可得 ab = -2p^2 (1)
l1的方程 y - a^2/(2p) = (a/p)(x -a) (2)
l1的方程 y - b^2/(2p) = (b/p)(x -b)(3)
由(2)(3): x = (a+b)/2
y = ab/(2p)= -2p^2/(2p) = -p
M的轨迹方程是y = -p