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 1.5 - Nested Quantifiers - Exercises - Page 68: 42

Answer

$\forall x \text{ }\forall y\text{ } \forall z\text{ }[(x+y)\cdot z=x\cdot z+y\cdot z]$ $\forall x \text{ }\forall y\text{ } \forall z\text{ }[x\cdot (y+z)=x\cdot y+x\cdot z]$

Work Step by Step

Our goal is to express the distributive laws of multiplication over addition using quantifiers, Let's consider the real numbers $x$, $y$, $z$. The two distributive laws of multiplication over addition are: $$\begin{align*} (x+y)\cdot z&=x\cdot z+y\cdot z\\ x\cdot (y+z)&=x\cdot y+x\cdot z. \end{align*}$$ Since these statements are true for any real numbers $x$, $y$, $z$, we will use the universal quantifier $\forall$ in front of each variable: $$\begin{align*} \forall x \text{ }\forall y\text{ } \forall z\text{ }[(x+y)\cdot z&=x\cdot z+y\cdot z]\\ \forall x \text{ }\forall y\text{ } \forall z\text{ }[x\cdot (y+z)&=x\cdot y+x\cdot z]. \end{align*}$$
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