what is a vertex angle

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I"m not sure what this has to do with quadratic equations, but a vertex is at the intersection of the sides of a polygon.  If you stand on the intersection point, looking into the polygon, the two rays left and right form the vertex angle.  In quadratic equations, the vertex is the turning point of the parabola with the coordinates of (-b/2a, y), but I don't know of any angle associated with that term.

It is the angle of the lowest point for a graph equation(s).