P(x,y)
p到点A(0,8)的距离比到直线y= —7的距离大1
√[(x^2+(y-8)^2]-|y+7|=1
√[(x^2+(y-8)^2]=1+|y+7|
x^2-30y+14=2|y+7|
y≥0,x^2=32y
y≥1,x^2=28(y-1)