M(x,y)与A的距离为√[x*2 + (y-2)^2]
直线y= -2可变为0x + y + 2 = 0,M与直线的距离:|0x + y + 2|/√(0^2 + 1^2) = |y+2|
|y+2| = √[x*2 + (y-2)^2]
平方:y^2 + 4y +4 = x^2 +y^2 -4y +4
x^2 = 8y