Lesson objective: Understand that triangles can be classified based on the angle measurements and identify triangles as acute, right, or obtuse triangles.

Students bring prior knowledge of angles from 4.MD.C.5. This prior knowledge is extended to classifying triangles by angle measurements as students sort triangles based on attributes. A conceptual challenge students may encounter is incorrectly classifying triangles due to incorrectly identifying the angles.

The concept is developed through work with real-world scenario, which allows the application of new skills to be put to use.

This work helps students deepen their understanding of equivalence because triangles may be classified according to their angle measure. For example, triangles with one angle equal to 90° are right triangles.

Students engage in Mathematical Practice 6 (attend to precision) as they accurately and efficiently calculate the measure of angle degrees.

**Key vocabulary:**

- acute angle
- obtuse angle
- right angle
- triangle

**Special materials needed:**

- protractor (optional)