对称轴是x=2,则设解析式是:y=a(x-2)^2+h
(1,4);(5,0)代入得:
4=a(1-2)^2+h
0=a(5-2)^2+h
解得:a=-1/2,h=4.5
即解析式是:y=-1/2(x-2)^2+4.5=-0.5x^2+2x+2.5