x=0,y=0,O(0,0)
x=40,y=0 A(40,0)
顶点x=20,y=16
B(20,16)
y=a(x-20)^2+h
0=400a+h
16=h
a=-4/100=-1/25
y= (-1/25)* (x-20)^2 +16