4.G.A.2
Classify two-dimensional figures based on the presence or absence of parallel or perpendicular lines, or the presence or absence of angles of a specified size. Recognize right triangles as a category, and identify right triangles.
Students will learn how to classify triangles, given either their side lengths or their angles. Given side lengths, they'll identify whether a triangle is equilateral, isosceles, or scalene. Given angles, they'll identify whether a triangle is right, obtuse, or acute. Finally, students will consider how classification by side length and classification by angle are related. For example, have students draw a Venn diagram, with the left circle labeled isosceles, and the right circle labeled acute. Have students determine which of the regions are occupied, and which are empty. Do this for other pairs as well, such as obtuse and right.