Discussion
If f(x) = [2x], where [x] denotes the greatest integer function in x , then the image of {-2.3, 2.9} is
Answer: B
The image is {-5, 5}.
Double each element first, then take the greatest integer not exceeding the result:
\[ \begin{aligned} f(-2.3) &= [-4.6] = -5 \\ f(2.9) &= [5.8] = 5 \end{aligned} \]
The trap is the negative value: the greatest integer not exceeding -4.6 is -5, not -4, because -4 is larger than -4.6. That rules out {-4, 5} and {-4, 6}. {-5, 3} and {-3, 2} each get the value at 2.9 wrong, since 2 times 2.9 is 5.8, whose integer part is 5.
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