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Multiplication by a positive number preserves the order: For a > 0, if b > c, then ab > ac. Multiplication by a negative number reverses the order: For a < 0, if b > c, then ab < ac. The complex numbers do not have an ordering that is compatible with both addition and multiplication. [30]
A multiplication by a negative number can be seen as a change of direction of the vector of magnitude equal to the absolute value of the product of the factors. When multiplying numbers, the magnitude of the product is always just the product of the two magnitudes. The sign of the product is determined by the following rules:
In the middle part, the text 2 × (-4) is written in black, except for the negative sign, which is blue. Below, there is a black number line partially graduated from -8 to 8. The first blue arc shaped arrow jumps from 1 to -4. Above it is the text 1 × (-4) . An identical arrow jumps from -4 to -8, and the text 2 × (-4) is written above.
For example, in the real numbers, the squaring operation only produces non-negative numbers; the codomain is the set of real numbers, but the range is the non-negative numbers. Operations can involve dissimilar objects: a vector can be multiplied by a scalar to form another vector (an operation known as scalar multiplication ), [ 13 ] and the ...
The sum of two numbers is unique; there is only one correct answer for a sums. [8] When the sum of a pair of digits results in a two-digit number, the "tens" digit is referred to as the "carry digit". [9] In elementary arithmetic, students typically learn to add whole numbers and may also learn about topics such as negative numbers and fractions.
Because of the rules of division of signed numbers (which states in part that negative divided by positive is negative), − 1 / 2 , −1 / 2 and 1 / −2 all represent the same fraction – negative one-half. And because a negative divided by a negative produces a positive, −1 / −2 represents positive one-half.
The whole numbers were synonymous with the integers up until the early 1950s. [23] [24] [25] In the late 1950s, as part of the New Math movement, [26] American elementary school teachers began teaching that whole numbers referred to the natural numbers, excluding negative numbers, while integer included the negative numbers.
To teach integer addition and subtraction, a number line is often used. A typical positive/negative number line spans from −20 to 20. A typical positive/negative number line spans from −20 to 20. For a problem such as “−15 + 17”, students are told to “find −15 and count 17 spaces to the right”.