y2=+/-2px焦点为(+/- P/2,0)准线为X=-/+ P/2
x2=+/-2py焦点为(0,+/- P/2)准线为Y=-/+ P/2
当焦点在X轴,焦点到准线的距离是2,对应方程为Y2=+/-2X
焦点到准线的距离为P