Читать книгу SAT Math For Dummies with Online Practice - Mark Zegarelli - Страница 40

The real numbers are the combined set of both rational and irrational numbers. This set includes every point on the number line.

Like the rational numbers, the set of real numbers is closed under the basic four operations. However, the set of real numbers isn’t closed under the operation of taking a square root, because the square root of a negative number isn’t a real number. For example:

The square root of a negative number is called an imaginary number — that is, a real number multiplied by .

Only a few SAT questions include imaginary numbers. You learn more about imaginary numbers in Chapter 12, where I focus on the roots of quadratic functions, and in Chapter 15, where I discuss operations with imaginary numbers.