Perpendicular
Two lines, vectors, planes, etc., are said to be perpendicular if they meet at a Right Angle. In 
, twoVectors 
and 
are Perpendicular if their Dot Product
In
, a Line with Slope 
is Perpendicular to a Line with Slope
. Perpendicular objects are sometimes said to be ``orthogonal.''In the above figure, the Line Segment 
is perpendicular to the Line Segment 
. This relationshipis commonly denoted with a small Square at the vertex where perpendicular objects meet, as shown above, and isdenoted 
.
- Heesch Number
- Heesch's Problem
- Height
- Heilbronn Triangle Problem
- Heine-Borel Theorem
- Heine Hypergeometric Series
- Heisenberg Group
- Heisenberg Space
- Held Group
- Helen of Geometers
- Helicoid
- Helix
- Helly Number
- Helly's Theorem
- Helmholtz Differential Equation
- Helmholtz Differential Equation--Bipolar Coordinates
- Helmholtz Differential Equation--Cartesian Coordinates
- Helmholtz Differential Equation--Circular Cylindrical Coordinates
- Helmholtz Differential Equation--Confocal Ellipsoidal Coordinates
- Helmholtz Differential Equation--Confocal Paraboloidal Coordinates
- Helmholtz Differential Equation--Conical Coordinates
- Helmholtz Differential Equation--Elliptic Cylindrical Coordinates
- Helmholtz Differential Equation--Oblate Spheroidal Coordinates
- Helmholtz Differential Equation--Parabolic Coordinates
- Helmholtz Differential Equation--Parabolic Cylindrical Coordinates