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DM topic guide

Syllogisms

Decide what must follow from category or conditional premises.

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Syllogisms

Decide what must follow from category or conditional premises.

Methoda clear route through this question type
1

Translate each premise.

2

Draw the smallest set picture or arrow chain.

3

Test one conclusion at a time.

4

Search for a counterexample when the claim says must.

◎How to spot it

What will the question look like?

Short statements use all, some, no, if, only or unless, followed by conclusions to test.

✦Rules to remember

Keep these in view

  • Use only the given structure.
  • The contrapositive is valid; the converse is not.
  • Universal statements do not guarantee existence.
⚑Common traps

What tends to pull students off course

  • Assuming existence
  • Treating some as exactly one
  • Combining different people
See it in action

Syllogisms in context

All R are S and no S are T means no R are T. Reversing 'no R are T' to 'no T are R' is also safe because exclusion works both ways.

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