half plane:   the set of all points in a plane that lie on one side of a line in the plane.

harmonic sequence:   a sequence is a harmonic sequence if the reciprocals of the numbers

                                    in the sequence form an arithmetic sequence.

heptagon:    a polygon with seven sides.

Heron's formula:   a formula for the area of a triangle:
                                        _________________
                              A =  (s(s - a)(s - b)(s - c))

where a, b, and c are the lengths of the sides of the triangle, and

s is half the perimeter.

hexadecimal number:   a number written in base sixteen.

hexagon:  a polygon with six sides.

hexahedron:  a polyhedron with six faces.   A regular hexahedron is a cube.

hyperbola:   the set of all points in a plane such that the difference of the distances to two

fixed points is a constant.    The general equation for a hyperbola is

x²  -   y²    =    1
a²         b²

hyperbolic functions:   the hyperbolic functions are defined as follows:

hyperbolic cosine:   cosh x = (1/2)(e^x + e^-x)

hyperbolic sine:       sinh x = (1/2)(e^x - e^-x)

hyperbolic tangent:   tanh x =    sinh x
                                                     -------
                                                     cosh x

hypotenuse:  the longest side of a right triangle.   The side opposite the right angle in a

                     right triangle.

hypothesis:  a proposition that is being investigated, it has yet to be proven.

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