In mathematics, an identity is an equality relating one mathematical expression A to another mathematical expression B, such that A and B (which might contain some variables) produce the same value for all values of the variables within a certain range of validity.^[1] In other words, A = B is an identity if A and B define the same functions, and an identity is an equality between functions that are differently defined. For example, ${\displaystyle (a+b)^{2}=a^{2}+2ab+b^{2}}$ and ${\displaystyle \cos ^{2}\theta +\sin ^{2}\theta =1}$ are identities.^[1] Identities are sometimes indicated by the triple bar symbol ≡ instead of =, the equals sign.^[2] Formally, an identity is a universally quantified equality.

Common identities

Algebraic identities

Certain identities, such as ${\displaystyle a+0=a}$ and ${\displaystyle a+(-a)=0}$, form the basis of algebra,^[3] while other identities, such as ${\displaystyle (a+b)^{2}=a^{2}+2ab+b^{2}}$ and ${\displaystyle a^{2}-b^{2}=(a+b)(a-b)}$, can be useful in simplifying algebraic expressions and expanding them.^[4]

Trigonometric identities

Geometrically, trigonometric identities are identities involving certain functions of one or more angles.^[5] They are distinct from triangle identities, which are identities involving both angles and side lengths of a triangle. Only the former are covered in this article.

These identities are useful whenever expressions involving trigonometric functions need to be simplified. Another important application is the integration of non-trigonometric functions: a common technique which involves first using the substitution rule with a trigonometric function, and then simplifying the resulting integral with a trigonometric identity.

One of the most prominent examples of trigonometric identities involves the equation ${\displaystyle \sin ^{2}\theta +\cos ^{2}\theta =1,}$ which is true for all real values of ${\displaystyle \theta }$. On the other hand, the equation

${\displaystyle \cos \theta =1}$

is only true for certain values of ${\displaystyle \theta }$, not all. For example, this equation is true when ${\displaystyle \theta =0,}$ but false when ${\displaystyle \theta =2}$.

Another group of trigonometric identities concerns the so-called addition/subtraction formulas (e.g. the double-angle identity ${\displaystyle \sin(2\theta )=2\sin \theta \cos \theta }$, the addition formula for ${\displaystyle \tan(x+y)}$),^[2] which can be used to break down expressions of larger angles into those with smaller constituents.

Exponential identities

The following identities hold for all integer exponents, provided that the base is non-zero:

${\displaystyle {\begin{aligned}b^{m+n}&=b^{m}\cdot b^{n}\\(b^{m})^{n}&=b^{m\cdot n}\\(b\cdot c)^{n}&=b^{n}\cdot c^{n}\end{aligned}}}$

Unlike addition and multiplication, exponentiation is not commutative. For example, 2 + 3 = 3 + 2 = 5 and 2 · 3 = 3 · 2 = 6, but 2³ = 8 whereas 3² = 9.

Also unlike addition and multiplication, exponentiation is not associative either. For example, (2 + 3) + 4 = 2 + (3 + 4) = 9 and (2 · 3) · 4 = 2 · (3 · 4) = 24, but 2³ to the 4 is 8⁴ (or 4,096) whereas 2 to the 3⁴ is 2⁸¹ (or 2,417,851,639,229,258,349,412,352). When no parentheses are written, by convention the order is top-down, not bottom-up:

${\displaystyle b^{p^{q}}:=b^{(p^{q})},}$   whereas   ${\displaystyle (b^{p})^{q}=b^{p\cdot q}.}$

Logarithmic identities

Several important formulas, sometimes called logarithmic identities or log laws, relate logarithms to one another:^[a]

Product, quotient, power and root

The logarithm of a product is the sum of the logarithms of the numbers being multiplied; the logarithm of the ratio of two numbers is the difference of the logarithms. The logarithm of the pth power of a number is p times the logarithm of the number itself; the logarithm of a pth root is the logarithm of the number divided by p. The following table lists these identities with examples. Each of the identities can be derived after substitution of the logarithm definitions ${\displaystyle x=b^{\log _{b}x},}$ and/or ${\displaystyle y=b^{\log _{b}y},}$ in the left hand sides.

Zdroj:https://en.wikipedia.org?pojem=Identity_(mathematics)
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	Formula	Example
product	${\displaystyle \log _{b}(xy)=\log _{b}(x)+\log _{b}(y)}$