设直线x=ty-2t+1联立得 y²=4（ty-2t+1）∴y1+y2=4t 中点M纵坐标y0=（y1+y2)/2=2t x0=2t²-2t+1t=y0/2 代入 x0=2t²-2t+1得x=y²/2-y+1
