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All dogs have tails

In maths, or at least university maths, there are a lot of statements that go like this: "If ...., then ..." or "Every ..., has ...." or "Every ..., is ...". For example, "Every rectangle has opposite sides parallel", "If two numbers are even, then their sum is even", "Every subspace contains the zero vector", "If a matrix has all distinct eigenvalues, then it is diagonalisable". Many students when faced with statements like these automatically and unconsciously assume that it works both ways, especially when the subject matter is new to them. This post is about a way of helping students see the problem.

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