CCSS.Math.Content.HSG-CO.D.12
The standard
Make formal geometric constructions with a variety of tools and methods (compass and straightedge, string, reflective devices, paper folding, dynamic geometric software, etc.). Copying a segment; copying an angle; bisecting a segment; bisecting an angle; constructing perpendicular lines, including the perpendicular bisector of a line segment; and constructing a line parallel to a given line through a point not on the line.
Common Core State Standards for Mathematics
What this standard means
Students need to make exact geometric constructions, not just sketch them. They should use tools like a compass, straightedge, patty paper, string, or geometry software to copy segments and angles, bisect segments and angles, and build perpendicular or parallel lines.
Mastery looks like clean construction marks, correct tool use, and a clear reason why the construction works. Students often get stuck by measuring with rulers or protractors instead of constructing, losing compass width, or skipping arcs that show their process.
Ways to teach it
- Have students use compass and straightedge to construct a perpendicular bisector, then verify by measuring distances from three points on the bisector.
- Ask students to write: How is constructing an angle bisector different from estimating half an angle with a protractor?
- Give a 5-minute exit ticket: copy a segment and construct its perpendicular bisector, leaving all arcs visible.
- Show a simple floor plan and ask students where to place a light so it is the same distance from two doors.
Plan a lesson for CCSS.Math.Content.HSG-CO.D.12
Generate a complete lesson plan aligned to this standard, with objectives, activities, and materials. Free, no account needed.
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