A geometric figure is closed if the entire boundary of the figure is part of the figure.^{[1]}  

The clearest example of this is a graph of inequalities. Figure 1 is the graph of a system of inequalities. The points on the line y < 3  x is part of the boundary. However, since the inequality y < 3  x does not include the points on the line, the shaded geometric figure is not closed.  

In figure 2, the inequality y ≤ 3  x does include the points on the line, so the shaded geometric figure is closed. 
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