x^2=y
纵坐标y=4时,x^2=4,x=±2,A（±2,4）
x^2=y=2*1/2*y,焦点纵坐标1/2÷2=1/4, 即焦点（0,1/4)
点A与抛物线焦点的距离 = √{(xA-0)^2+(yA-1/4)^2} = √{2^2+(4-1/4)^2}= 17/4