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Why Read This Book? - Index of

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6 Chapter 0 Notation and Assumptions<br />

(A16) Trichotomy law: For any a ∈ R, exactly one <strong>of</strong> the following is true:<br />

(a) a>0, in which case we say a is positive<br />

(b) a = 0<br />

(c) 0 >a, in which case we say a is negative<br />

(A17) For all a, b ∈ R,ifa>0 and b>0, then a + b>0. That is, the set <strong>of</strong> positive<br />

real numbers is closed under addition.<br />

(A18) For all a, b ∈ R,ifa>0 and b>0, then ab > 0. That is, the set <strong>of</strong> positive<br />

real numbers is closed under multiplication.<br />

Now we can use A16–A18 to give meaning to other statements comparing any<br />

pair <strong>of</strong> real numbers.<br />

Definition 0.2.1 Given real numbers a and b, we say that a>bif a − b>0.<br />

The statement aa. The statement a ≥ b means that either a>bor<br />

a = b. Similarly, a ≤ b means either a

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