Direct Proof and Counterexample I:Introduction

•To disprove a statement means to show that it is false. •Showing a statement is false is equivalent to showing that its negation is true. •So, disprove ∀x in D, P(x). is equivalent to prove ∃x in D such that ~P(x). which can be proved giving constructing an example element of D that makes P(x) false (i.e., constructive proofs of ... ................
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