Discrete Mathematics and Its Applications, Seventh Edition

Published by McGraw-Hill Education
ISBN 10: 0073383090
ISBN 13: 978-0-07338-309-5

Chapter 1 - Section 1.7 - Introduction to Proofs - Exercises - Page 91: 25

Answer

. Use a proof by contradiction to show that there is no rational number r for which r^3 + r + 1 = 0. [Hint: Assume that r = a/b is a root, where a and b are integers and a/b is in lowest terms. Obtain an equation involving integers by multiplying by b^3. Then look at whether a and b are each odd or even.]

Work Step by Step

So from figure 1 and 2 we see that the actual statement is true. Hence it is proved that there is no rational number r for which r^3+r+1 = 0
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