Term

Definition

the time it takes for a substance to decay to half its original weight

a positive integer whose digits, when squared, add to 1
23 ==> 2² + 3² = 4 + 9 = 13 ==> 1² + 3² = 1 + 9 = 10 ==> 1² + 0² = 1  ∴ 23 is a happy number
44 ==> 4² + 4² = 16 + 16 = 32 ==> 3² + 2²= 9 + 4 ==> 13 ==> 1² + 3² = 1 + 9 = 10 ==> 1² + 0² = 1  ∴ 44 is a happy number
391 ==> 3² + 9² + 1² = 9 + 81 + 1 = 91 ==> 9² + 1² = 82 ==> 8² + 2² = 64 + 4 = 68 ==> 6² + 8² = 36 + 64 = 100 ==> = 1² + 0² + 0² = 1  ∴ 391 is a happy number

Highest Common Factor, the highest number that fits evenly into two or more numbers

area that is equal to 10 000 m², with the abbreviation ha.  There are 100 hectares in 1 km².

a polygon with 7 sides

used to find the area of a triangle when three sides are known. $$A=\sqrt$$, where $$s=\frac$$ S is also known as the semi-perimeter of the triangle

a polygon with 6 sides

a solid with six faces. A regular hexahedron is a cube – all six faces the same.

a flat line or plane that runs parallel to the horizon

when a horizontal line is drawn through a function if that line only intersects with the function once, it is a one-to-one function and will have a naturally occurring inverse function

the most basic hyperbola is the function $$y=\frac$$