Sign of a quadratic function

MathematicsAlgebraAges 15–16

Drag a, b and c to change the parabola y = ax² + bx + c and read the matching sign table right away. The simulation shades the part of the graph above the x-axis in green and the part below in red, so it becomes clear why the sign of f(x) depends only on the sign of a and on how the discriminant Δ compares with 0.