The equation of a plane in normal form (with unit normal n̂ and perpendicular distance d from origin) is

1. In normal form, n̂ is a unit normal vector and d is the perpendicular distance from the origin. 2. For any point r on the plane, its projection on n̂ equals d. 3. The projection is the dot product r · n̂. 4. So the plane equation is r · n̂ = d. _Source: NCERT Class 12 Maths Part 2 Ch 11 "Three Dimensional Geometry", §11.6.1_