Answer
The set of irrational numbers only
Work Step by Step
$\sqrt 21$ is an unending decimal, so it must only be an irrational number.
Algebra 1: Common Core (15th Edition)
by Charles, Randall I.
Published by
Prentice Hall
ISBN 10:
0133281140
ISBN 13:
978-0-13328-114-9
Chapter 1 - Foundations for Algebra - 1-5 Adding and Subtracting Real Numbers - Practice and Problem-Solving Exercises - Page 36: 82
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