In classic geometry, it opens down when the directrix is above the focus.
In analytical (coordinate) geometry, if the equation of the parabola is
y = ax^2 + bx + c, it opens down if a < 0.
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A parabola can open left, down, right, or left on a graph, if that's what you mean:\
positive.
This is called the 'standard form' for the equation of a parabola:y =a (x-h)2+vDepending on whether the constant a is positive or negative, the parabola will open up or down.
If a is greater than zero then the parabola opens upward.
To find the value of a in a parabola opening up or down subtract the y-value of the parabola at the vertex from the y-value of the point on the parabola that is one unit to the right of the vertex.