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Published byHannah Palmer Modified over 5 years ago

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Can you think of five famous people who you feel are rational/logical?

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Deciding whether a syllogism is valid Trying to decide if a syllogism is valid is not easy. Venn diagrams can help (at last a good use for Venn diagrams!)

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Deciding whether a syllogism is valid Some IB students are from Russia All Russians are good at drinking vodka Therefore some IB students are good at drinking vodka Is this a valid argument? Mmmmmm m…Vodka!

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Using Venn diagrams Some IB students are from Russia IB Students Russians IB Students who are Russian

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Using Venn diagrams All Russians are good at drinking vodka IB Students Russians Good vodka drinkers

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Using Venn diagrams Therefore some IB students are good at drinking vodka IB Students Russians Good vodka drinkers

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Another example All As are Bs All Bs are Cs Therefore all Cs are As

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Another example All As are Bs A B

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Another example All Bs are Cs A B C

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Another example Therefore all Cs are As A B C These Cs are not As The syllogism is not valid

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Another example! All As are Bs Some As are Cs Therefore some Bs are Cs

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Another example! All As are Bs A B

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Another example! Some As are Cs A B C

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Another example! Therefore some Bs are Cs A B C The syllogism is valid

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Now your turn Using Venn diagrams in your notebooks, decide whether each of the following arguments is valid or invalid.

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Valid or invalid All Norwegians eat hotdogs Edward eats hotdogs Therefore Edward is Norwegian

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Valid or invalid No Swedes have red noses Mr Luitjens has a red nose Therefore Mr Luitjens is not a Swede

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Valid or invalid All IB1 boys are brave Some brave people are compassionate Some IB1 boys are compassionate

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Valid or invalid Some physicists are frauds Some frauds are not wealthy Therefore some physicists are not wealthy

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Valid or invalid All Norwegians have dogs No good football players have dogs Therefore no Norwegians are good football players

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Valid or invalid All clever girls are red-headed All red heads are rich Therefore all clever girls are rich

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Valid or invalid No teachers are clever No IB1 students are clever Therefore no IB1 students are teachers

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