就是定义转化.
抛物线是:y^2=2px
则为p+x1+x2
抛物线是:x^2=2py
则为p+y1+y2
抛物线是:y^2=-2px
则为p-x1-x2
抛物线是:x^2=-2py
则为p-y1-y2