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In mathematics, the additive inverse of a number a (sometimes called the opposite of a)[1] is the number that, when added to a, yields zero. The operation taking a number to its additive inverse is known as sign change[2] or negation.[3] For a real number, it reverses its sign: the additive inverse (opposite number) of a positive number is negative, and the additive inverse of a negative number is positive. Zero is the additive inverse of itself.

The additive inverse of a is denoted by unary minus: a (see also § Relation to subtraction below).[4] For example, the additive inverse of 7 is −7, because 7 + (−7) = 0, and the additive inverse of −0.3 is 0.3, because −0.3 + 0.3 = 0.

Similarly, the additive inverse of ab is −(ab) which can be simplified to ba. The additive inverse of 2x − 3 is 3 − 2x, because 2x − 3 + 3 − 2x = 0.[5]

The additive inverse is defined as its inverse element under the binary operation of addition (see also § Formal definition below), which allows a broad generalization to mathematical objects other than numbers. As for any inverse operation, double additive inverse has no net effect: −(−x) = x.

These complex numbers, two of eight values of 81, are mutually opposite

Common examples

For a number (and more generally in any ring), the additive inverse can be calculated using multiplication by −1; that is, n = −1 × n. Examples of rings of numbers are integers, rational numbers, real numbers, and complex numbers.

Relation to subtraction

Additive inverse is closely related to subtraction, which can be viewed as an addition of the opposite:

ab  =  a + (−b).

Conversely, additive inverse can be thought of as subtraction from zero:

a = 0 − a.

Hence, unary minus sign notation can be seen as a shorthand for subtraction (with the "0" symbol omitted), although in a correct typography, there should be no space after unary "−".

Other properties

In addition to the identities listed above, negation has the following algebraic properties:

Formal definition

The notation + is usually reserved for commutative binary operations (operations where x + y = y + x for all x, y). If such an operation admits an identity element o (such that x + o ( = o + x ) = x for all x), then this element is unique (o = o + o = o). For a given x, if there exists x such that x + x ( = x + x ) = o, then x is called an additive inverse of x.

If + is associative, i.e., (x + y) + z = x + (y + z) for all x, y, z, then an additive inverse is unique. To see this, let x and x″ each be additive inverses of x; then

x = x + o = x + (x + x″) = (x + x) + x″ = o + x″ = x″.

For example, since addition of real numbers is associative, each real number has a unique additive inverse.

Other examples

All the following examples are in fact abelian groups:


Natural numbers, cardinal numbers and ordinal numbers do not have additive inverses within their respective sets. Thus one can say, for example, that natural numbers do have additive inverses, but because these additive inverses are not themselves natural numbers, the set of natural numbers is not closed under taking additive inverses.

See also

Notes and references

  1. ^ Tussy, Alan; Gustafson, R. (2012), Elementary Algebra (5th ed.), Cengage Learning, p. 40, ISBN 9781133710790.
  2. ^ Brase, Corrinne Pellillo; Brase, Charles Henry (1976). Basic Algebra for College Students. Houghton Mifflin. p. 54. ISBN 978-0-395-20656-0. take the additive inverse of the member, we change the sign of the number.
  3. ^ The term "negation" bears a reference to negative numbers, which can be misleading, because the additive inverse of a negative number is positive.
  4. ^ Weisstein, Eric W. "Additive Inverse". Retrieved 2020-08-27.
  5. ^ "Additive Inverse". Retrieved 2020-08-27.