The graph of a quadratic function y = ax² + bx + c is called a parabola. The following graphs are two typical parabolas:

For the sake of explanation, we mark their x-intercepts by red dots, their y-intercepts by a pink dot, and the vertex of each parabola by a green dot:

The first parabola opens upwards (is a U shape) and the second parabola opens downwards (is an upside down U shape). 

Given the function y = ax² + bx + c :

·       If a > 0 (positive) then the parabola opens upward.

·       If a < 0 (negative) then the parabola opens downward.