Dashed and Solid Boundary Lines
When graphing a linear inequality in two variables, the boundary line is drawn either dashed or solid to show whether points on the line itself belong to the solution set. This convention parallels the parenthesis/bracket notation used for one-variable inequalities on a number line:
- Dashed line: For strict inequalities ( or ), the boundary line is not included in the solution. It is drawn as a dashed line.
- Solid line: For non-strict inequalities ( or ), the boundary line is included in the solution. It is drawn as a solid line.
For example, the graph of uses a dashed boundary line at because the strict inequality symbol means that points satisfying exactly are not part of the solution set.
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