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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. Mental calculation - Wikipedia

    en.wikipedia.org/wiki/Mental_calculation

    This technique allows easy multiplication of numbers close and below 100.(90-99) [2] The variables will be the two numbers one multiplies. The product of two variables ranging from 90-99 will result in a 4-digit number. The first step is to find the ones-digit and the tens digit. Subtract both variables from 100 which will result in 2 one-digit ...

  4. Cross product - Wikipedia

    en.wikipedia.org/wiki/Cross_product

    The cross product with respect to a right-handed coordinate system. In mathematics, the cross product or vector product (occasionally directed area product, to emphasize its geometric significance) is a binary operation on two vectors in a three-dimensional oriented Euclidean vector space (named here ), and is denoted by the symbol .

  5. Order of operations - Wikipedia

    en.wikipedia.org/wiki/Order_of_operations

    [2] [3] Thus, in the expression 1 + 2 × 3, the multiplication is performed before addition, and the expression has the value 1 + (2 × 3) = 7, and not (1 + 2) × 3 = 9. When exponents were introduced in the 16th and 17th centuries, they were given precedence over both addition and multiplication and placed as a superscript to the right of ...

  6. Algebraic expression - Wikipedia

    en.wikipedia.org/wiki/Algebraic_expression

    In mathematics, an algebraic expression is an expression built up from constants (usually, algebraic numbers) variables, and the basic algebraic operations: addition (+), subtraction (-), multiplication (×), division (÷), whole number powers, and roots (fractional powers).

  7. Dyadics - Wikipedia

    en.wikipedia.org/wiki/Dyadics

    In this sense, the unit dyadic ij is the function from 3-space to itself sending a 1 i + a 2 j + a 3 k to a 2 i, and jj sends this sum to a 2 j. Now it is revealed in what (precise) sense ii + jj + kk is the identity: it sends a 1 i + a 2 j + a 3 k to itself because its effect is to sum each unit vector in the standard basis scaled by the ...

  8. Rod calculus - Wikipedia

    en.wikipedia.org/wiki/Rod_calculus

    Cross multiply 1 with 5, 2 with 3 to get 5 and 6, replace the numerators with the corresponding cross products. Multiply the two denominators 3 × 5 = 15, put at bottom right; Add the two numerators 5 and 6 = 11 put on top right of counting board. Result: ⁠ 1 / 3 ⁠ + ⁠ 2 / 5 ⁠ = ⁠ 11 / 15 ⁠

  9. Heaviside cover-up method - Wikipedia

    en.wikipedia.org/wiki/Heaviside_cover-up_method

    When a partial fraction term has a single (i.e. unrepeated) binomial in the denominator, the numerator is a residue of the function defined by the input fraction. We calculate each respective numerator by (1) taking the root of the denominator (i.e. the value of x that makes the denominator zero) and (2) then substituting this root into the ...