与x轴交于A（1,0）,B（5,0）,可写作y=a(x-1)(x-5)=ax^2-6ax+5a=a(x-3)^2-4a
顶点P（3,-4a)
PB=2√5
(3-5)^2+(-4a)^2 = (2√5)^2
a^2=1
a=±1
y = -x^2+6x-5；或y = x^2-6x+5