NY Next Generation Math NY-4.G.2.a

Math4th GradeDraw and identify lines and angles, and classify shapes by properties of their lines and angles.

The Standard

Identify and name triangles based on angle size (right, obtuse, acute).

New York State Next Generation Mathematics Learning Standards

Teacher's field guide

What Students Need to Do: Students classify triangles by the sizes of their angles. They inspect all three angles before naming a triangle as acute, right, or obtuse.
What Mastery Looks Like: A student names a triangle with one 90-degree angle as right and one with an angle greater than 90 degrees as obtuse. The student knows an acute triangle has three acute angles.
Common Misconceptions: Students may classify a triangle as acute because it has one acute angle, even when another angle is right or obtuse. They may also sort only by side length.
How to Assess It: Show three unlabeled triangles and ask students to classify each by angle size, mark the deciding angle or angles, and explain their choices.

Lesson moves

Build triangles with hinged craft sticks, then adjust one angle and record when the triangle becomes acute, right, or obtuse.
Ask students why a right triangle cannot also be an obtuse triangle, even though both contain acute angles.
Play triangle sort by drawing a card, naming its angle class, and placing it under the matching header.
Photograph triangular signs or roof frames and classify each triangle by its angles.

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