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\leftline{Mathematics 242 -- Principles of Analysis}
\leftline{Definitions Quiz 7 -- April 12, 2013}
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\leftline{Your Name: $\underline{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }$}
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A) (5)  What does it mean to say that a set is {\it countably infinite}?   
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B) (5)  Let 
$$f(x) = \cases{1-3x & if $x < 0$ \cr
                1+2x & if $x \ge 0$.\cr}$$
Is $f$ differentiable at $c = 0$?  Why or why not?  
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