One half

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½
prefixes (from Greek) / (from Latin)
Binary 0.1 or 0.011111111111...
Decimal 0.5 or 0.499999999999...
Hexadecimal 0.8 or 0.7FFFFFFFFFFF...
Continued fraction [0; 1, 1] or [0; 2]
Single-precision floating point 3F000000 (hex) = 00111111000000000000000000000000 (binary)
One half is the irreducible fraction resulting from dividing one by two (½), or any number by its double; multiplication by one half is equivalent to division by two. It is the fraction occurring most often in mathematical equations, recipes, measurements, etc.

For instance, the area S of a triangle is computed

S = ½ × base × perpendicular height

One half also figures in the formula for calculating figurate numbers, such as triangular numbers and pentagonal numbers:

½ × n ( ( s - 2 ) n - ( 4 - s ) )

and in the formula for computing magic constants for magic squares

M₂(n) = ½ × ( n ( n² + 1 ) )

One half has two different decimal expansions, the familiar 0.5 and the recurring 0.49999999... It has a similar pair of expansions in any even base. It's a common trap to believe these expressions represent distinct numbers: see the proof that 0.999... equals 1 for detailed discussion of a related case.

One half is also:

One of the few fractions to get a key of its own on typewriters. It also gets its own point in some early extensions of ASCII at 171; and in Unicode, it gets its own code point at 189 in the C1 Controls and Latin-1 Supplement block, and a cross-reference in the Number Forms block, which contains some other fractions.
One of the few fractions which is commonly expressed in natural languages by suppletion rather than regular derivation; compare English one half with regular formations like one sixth from six.

½
prefixes	(from Greek) / (from Latin)
Binary	0.1 or 0.011111111111...
Decimal	0.5 or 0.499999999999...
Hexadecimal	0.8 or 0.7FFFFFFFFFFF...
Continued fraction	[0; 1, 1] or [0; 2]
Single-precision floating point	3F000000 (hex) = 00111111000000000000000000000000 (binary)

One half

See also