Dear Dr. Wicklin: After thinking it over again, I think I made a mistake in my previous message regarding the analogy. In my problem, the limit is also a constant: It is nothing but 0.5. The target is for f*x to approach the limit of 0.5 as closely as possible from TWO directions. Here, the limit of 0.5 corresponds to the maximum weight the backpack can hold in Example 22.3. It is just that, in Example 22.3, it is only in ONE direction. Still, thank you for reminding me of the use of the absolute value operation. Thanks a lot!
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