Operations research
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3 graphical LP mistakes that cost marks
3 mistakes that cost marks in graphical LP. Are you making one?
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Lesson notes
Plot constraints, identify the feasible region and find the optimum at a corner point
Key points
- Shading the wrong side: For carpentry hours ≤ 40, shade toward the origin; test point (0,0) to check every constraint.
- Skipping corner points: Students evaluate one or two vertices; list every corner of the feasible region and compute profit at each.
- Optimum inside region: An LP optimum is never only interior; it lies at a vertex, or along an edge for multiple optima.
Takeaway: Shade correctly, check every corner, optimum sits on the boundary.
Quick check: Where does a bounded two-variable LP's optimum lie?
- At the region's centre
- At a corner point
- Wherever lines are steepest
Linear objectives reach their best value at a vertex of the feasible region.
feasible regioncorner pointiso-profit line
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