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Standard 6: Attend to Precision

Clip 2/2: Students Define Geometric Terms


Mathematically proficient students try to communicate precisely to others. They try to use clear definitions in discussion with others and in their own reasoning.

At the beginning of the lesson, as students describe the prior lesson, they demonstrate precision of mathematical language in their use of geometric terms when categorizing triangles, and in defining those terms (acute, right, obtuse, equilateral, isosceles, and scalene) in their own words. Some examples include:

  • “Acute is an angle that is less than 90 degrees.”
  • “Obtuse is an angle that is more than 90 degrees.”
  • “A right angle is exactly 90 degrees.”
  • “It's like a right angle because see how number E on the chart, it's in the right angle category? If you tilt it more to the left then it still looks like the corner of a square box...”

See this video in the context of an entire lesson: Classifying Triangles

This clip also relates to Standard 7 (look for and make use of structure).

Materials & Artifacts