Wolfram Research
Wolfram Math World: Plane
This site defines a plane in terms of geometric surfaces. Content includes formulas for a plane using the normal vector, how to find the x, y, and z intercepts of a plane, distance from the origin, point plane distance formula, dihedral...
CK-12 Foundation
Ck 12: Algebra: Quadratic Functions and Their Graphs
[Free Registration/Login may be required to access all resource tools.] This lesson covers how to find the x-intercepts, vertex, axis of symmetry, and y-intercept of a parabola.