One of the question from my text book, it give a theorem and says that prove it as a direct theorem. For two statements A and B, the direct theorem is "if A is true, then B is true." In this case, is it possible to prove this theorem by using contradiction?? Or do I must assume that A is true and conclude B is true?
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1It may be possible to prove it by contradiction, but the question is asking you to prove it directly. Start by assuming A, and see where that takes you – David Steinberg Feb 07 '14 at 18:48
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When you have to proof $A\implies B$, you can do it with a direct proof, a proof by contradiction (and some other ways). A direct proof indeed assumes that $A$ is true and from that you should proof (conclude) that $B$ is true. A proof by contradiction assumes $B$ is false and deduces that $A$ must be false too then.
Ragnar
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