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## Summary:

- Author seems to use both in similar/identical context.

I found a really nice set of lectures by Herb Gross on the subject of calculus, but I'm confused by his usage of

It seems to me that he describes identical situations on both pages, but in one case the author uses the wording "if and only if" while on the other page, an "if ... then" expression is found. Is he using them interchangeably here? Why did he change the wording the second time around? I need to make sure I understand his usage of underlined (red) expressions otherwise I won't be able to follow more elaborate arguments that will follow.

*if and only*if and*if ... then*in his supplementary notes.It seems to me that he describes identical situations on both pages, but in one case the author uses the wording "if and only if" while on the other page, an "if ... then" expression is found. Is he using them interchangeably here? Why did he change the wording the second time around? I need to make sure I understand his usage of underlined (red) expressions otherwise I won't be able to follow more elaborate arguments that will follow.

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