Discrete Mathematics with Applications 4th Edition

Published by Cengage Learning
ISBN 10: 0-49539-132-8
ISBN 13: 978-0-49539-132-6

Chapter 4 - Elementary Number Theory and Methods of Proof - Exercise Set 4.1 - Page 161: 14

Answer

The statement $(a+b)^{2}=a^{2}+b^{2}$ is true for some integers but false for others.

Work Step by Step

The simple case $a=b=0$ shows that the statement is sometimes true, since $(0+0)^{2}=0=0^{2}+0^{2}$. However, cases like $a=1$ and $b=2$ falsify the statement, since $(1+2)^{2}=3^{2}=9$, but $1^{2}+2^{2}=1+4=5$. In fact, since the distributive property (with some simplification) tells us that $(a+b)^{2}=a^{2}+2ab+b^{2}$, the given statement holds if and only if $a^{2}+b^{2}$$=a^{2}+2ab+b^{2}$, i.e., if $2ab=0$, so that $a=0$ or $b=0$.
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