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2. Cross-multiplication - Wikipedia

   en.wikipedia.org/wiki/Cross-multiplication

   where x is a variable we are interested in solving for, we can use cross-multiplication to determine that x = b c d . {\displaystyle x={\frac {bc}{d}}.} For example, suppose we want to know how far a car will travel in 7 hours, if we know that its speed is constant and that it already travelled 90 miles in the last 3 hours.

3. Nested radical - Wikipedia

   en.wikipedia.org/wiki/Nested_radical

   In the case of two nested square roots, the following theorem completely solves the problem of denesting. [2]If a and c are rational numbers and c is not the square of a rational number, there are two rational numbers x and y such that + = if and only if is the square of a rational number d.

4. Algebraic expression - Wikipedia

   en.wikipedia.org/wiki/Algebraic_expression

   A rational algebraic expression (or rational expression) is an algebraic expression that can be written as a quotient of polynomials, such as x 2 + 4x + 4. An irrational algebraic expression is one that is not rational, such as √ x + 4.

5. Thomae's function - Wikipedia

   en.wikipedia.org/wiki/Thomae's_function

   Every countable subset of the real numbers - such as the rational numbers - has measure zero, so the above discussion shows that Thomae's function is Riemann integrable on any interval. The function's integral is equal to 0 {\displaystyle 0} over any set because the function is equal to zero almost everywhere .

6. Expression (mathematics) - Wikipedia

   en.wikipedia.org/wiki/Expression_(mathematics)

   For a given combination of values for the free variables, an expression may be evaluated, although for some combinations of values of the free variables, the value of the expression may be undefined. Thus an expression represents an operation over constants and free variables and whose output is the resulting value of the expression. [22]

7. Commensurability (mathematics) - Wikipedia

   en.wikipedia.org/wiki/Commensurability_(mathematics)

   Example: Let a and b be nonzero real numbers. Then the subgroup of the real numbers R generated by a is commensurable with the subgroup generated by b if and only if the real numbers a and b are commensurable, in the sense that a/b is rational. Thus the group-theoretic notion of commensurability generalizes the concept for real numbers.

8. Indicator function - Wikipedia

   en.wikipedia.org/wiki/Indicator_function

   The notation is also used to denote the characteristic function in convex analysis, which is defined as if using the reciprocal of the standard definition of the indicator function.

9. Rational number - Wikipedia

   en.wikipedia.org/wiki/Rational_number

   In mathematics, "rational" is often used as a noun abbreviating "rational number". The adjective rational sometimes means that the coefficients are rational numbers. For example, a rational point is a point with rational coordinates (i.e., a point whose coordinates are rational numbers); a rational matrix is a matrix of rational numbers; a rational polynomial may be a polynomial with rational ...