EXAMPLE2 命题逻辑 Consider the following sentences. What time is it? 2. Read this carefully 3.X+1=2 4. x+y=Z Sentences I and 2 are not propositions because they are not statements. Sentences 3 and 4 are not propositions because they are neither tree nor false, since the variables in these sentences have not been assigned values. various ways to form propositions from sentences of this type will be discussed in Section 1.3 2/24/202111:37PM Deren Chen, ZheJiang UnivL o g i c 命题逻辑 2/24/2021 11:37 PM Deren Chen, ZheJiang Univ. 7 EXAMPLE 2 Consider the following sentences. 1. What time is it? 2. Read this carefully. 3. x+1 =2. 4. x+y = z. Sentences 1 and 2 are not propositions because they are not statements. Sentences 3 and 4 are not propositions because they are neither tree nor false, since the variables in these sentences have not been assigned values. Various ways to form propositions from sentences of this type will be discussed in Section 1.3