As shown in the figure, the parabola has the opening downward and the vertex as the origin.

Let y = ax 2, a.

When y=OC=-0.8, x 1-x2=AB=4.

So: AC=BC=4/2=2.

X=2，y=-0.8:

y=4a=-0.8

Solution: a=-0.2

So the analytical formula of parabola is y =-0.2x 2.