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Which region does ∣x−y∣≤1/4|x-y|\le 1/4 correspond to in the unit square?

The inequality ∣x−y∣≤1/4|x-y|\le 1/4 corresponds to a diagonal band-shaped region centered on the line y=xy=x. It is bounded by the two parallel lines y−x=1/4y-x=1/4 and x−y=1/4x-y=1/4 (or y=x+1/4y=x+1/4 and y=x−1/4y=x-1/4) within the unit square.

Conditions

  • The region is plotted within the unit square Ω=[0,1]×[0,1]\Omega=[0,1]\times[0,1].
  • The axes represent arrival times xx and yy.

Reasoning, step by step

  1. Rewrite ∣x−y∣≤1/4|x-y|\le 1/4 as −1/4≤x−y≤1/4-1/4 \le x-y \le 1/4.
  2. Identify the boundary lines x−y=1/4x-y=1/4 and x−y=−1/4x-y=-1/4.
  3. Visualize the strip between these lines inside the square.
  4. Note that this strip represents all pairs (x,y)(x,y) where the time difference is small enough to meet.

Example

The script states: 'The inequality ∣x−y∣≤1/4|x-y|\le1/4 is equivalent to −1/4≤x−y≤1/4-1/4\le x-y\le1/4. Inside the unit square, it forms the red band between y−x=1/4y-x=1/4 and x−y=1/4x-y=1/4.'

Common misconceptions

  • Thinking it is a triangular region.
  • Confusing the boundaries with the axes.
  • Assuming the band covers the entire square.

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