Algebraic expression

In mathematics, an algebraic expression is an expression built up from constant algebraic numbers, variables, and the algebraic operations (addition, subtraction, multiplication, division and exponentiation by an exponent that is a rational number). For example, 3x² − 2xy + c is an algebraic expression. Since taking the square root is the same as raising to the power 1/2, the following is also an algebraic expression:

${\sqrt {\frac {1-x^{2}}{1+x^{2}}}}$

An algebraic equation is an equation involving only algebraic expressions.

By contrast, transcendental numbers like π and e are not algebraic, since they are not derived from integer constants and algebraic operations. Usually, π is constructed as a geometric relationship, and the definition of e requires an infinite number of algebraic operations.

A rational expression is an expression that may be rewritten to a rational fraction by using the properties of the arithmetic operations (commutative properties and associative properties of addition and multiplication, distributive property and rules for the operations on the fractions). In other words, a rational expression is an expression which may be constructed from the variables and the constants by using only the four operations of arithmetic. Thus,

${\frac {3x-2xy+c}{y-1}}$

is a rational expression, whereas

${\sqrt {\frac {1-x^{2}}{1+x^{2}}}}$

is not, i.e. this is an irrational expression.

A rational equation is an equation in which two rational fractions (or rational expressions) of the form

${\frac {P(x)}{Q(x)}}$

are set equal to each other. These expressions obey the same rules as fractions. The equations can be solved by cross-multiplying. Division by zero is undefined, so that a solution causing formal division by zero is rejected.

Terminology

Algebra has its own terminology to describe parts of an expression:

1 – Exponent (power), 2 – coefficient, 3 – term, 4 – operator, 5 – constant, $x,y$ - variables

In roots of polynomials

The roots of a polynomial expression of degree n, or equivalently the solutions of a polynomial equation, can always be written as algebraic expressions if n < 5 (see quadratic formula, cubic function, and quartic equation). Such a solution of an equation is called an algebraic solution. But the Abel–Ruffini theorem states that algebraic solutions do not exist for all such equations (just for some of them) if n $\geq$ 5.

Conventions

Variables

By convention, letters at the beginning of the alphabet (e.g. $a,b,c$) are typically used to represent constants, and those toward the end of the alphabet (e.g. $x,y$ and $z$) are used to represent variables. They are usually written in italics.

Exponents

By convention, terms with the highest power (exponent), are written on the left, for example, $x^{2}$ is written to the left of $x$. When a coefficient is one, it is usually omitted (e.g. $1x^{2}$ is written $x^{2}$). Likewise when the exponent (power) is one, (e.g. $3x^{1}$ is written $3x$), and, when the exponent is zero, the result is always 1 (e.g. $3x^{0}$ is written $3$, since $x^{0}$ is always $1$).

Algebraic and other mathematical expressions

The table below summarizes how algebraic expressions compare with several other types of mathematical expressions by the type of elements they may contain, according to common but not universal conventions.

	Arithmetic expressions	Polynomial expressions	Algebraic expressions	Closed-form expressions	Analytic expressions	Mathematical expressions
Constant	Yes	Yes	Yes	Yes	Yes	Yes
Elementary arithmetic operation	Yes	Addition, subtraction, and multiplication only	Yes	Yes	Yes	Yes
Finite sum	Yes	Yes	Yes	Yes	Yes	Yes
Finite product	Yes	Yes	Yes	Yes	Yes	Yes
Finite continued fraction	Yes	No	Yes	Yes	Yes	Yes
Variable	No	Yes	Yes	Yes	Yes	Yes
Integer exponent	No	Yes	Yes	Yes	Yes	Yes
Integer nth root	No	No	Yes	Yes	Yes	Yes
Rational exponent	No	No	Yes	Yes	Yes	Yes
Integer factorial	No	No	Yes	Yes	Yes	Yes
Irrational exponent	No	No	No	Yes	Yes	Yes
Exponential function	No	No	No	Yes	Yes	Yes
Logarithm	No	No	No	Yes	Yes	Yes
Trigonometric function	No	No	No	Yes	Yes	Yes
Inverse trigonometric function	No	No	No	Yes	Yes	Yes
Hyperbolic function	No	No	No	Yes	Yes	Yes
Inverse hyperbolic function	No	No	No	Yes	Yes	Yes
Root of a polynomial that is not an algebraic solution	No	No	No	No	Yes	Yes
Gamma function and factorial of a non-integer	No	No	No	No	Yes	Yes
Bessel function	No	No	No	No	Yes	Yes
Special function	No	No	No	No	Yes	Yes
Infinite sum (series) (including power series)	No	No	No	No	Convergent only	Yes
Infinite product	No	No	No	No	Convergent only	Yes
Infinite continued fraction	No	No	No	No	Convergent only	Yes
Limit	No	No	No	No	No	Yes
Derivative	No	No	No	No	No	Yes
Integral	No	No	No	No	No	Yes

A rational algebraic expression (or rational expression) is an algebraic expression that can be written as a quotient of polynomials, such as x² + 4x + 4. An irrational algebraic expression is one that is not rational, such as √x + 4.

See also

• Algebraic equation
• Algebraic function
• Analytical expression
• Arithmetic expression
• Closed-form expression
• Expression (mathematics)
• Precalculus
• Polynomial
• Term (logic)

References

• James, Robert Clarke; James, Glenn (1992). Mathematics dictionary. p. 8. ISBN 9780412990410.

External links

• Weisstein, Eric W. "Algebraic Expression". MathWorld.