In this lesson, students examine sets of triangles in which all the triangles share 3 common measures of angles or sides. Students learn to recognize when triangles are “identical copies” that are oriented differently on the page, and when they are different triangles (meaning triangles that are not identical copies). This prepares them for trying to draw more than one triangle given 3 measures, in the next lesson.

For example, suppose a triangle has angles that measure and and a side length that measures . Here are 3 triangles that have these measures:

This example shows 2 “different triangles” (triangles that are not identical copies). The first two triangles are identical copies, so they are the same, but the third is not, so it is different from the other two.

Students construct arguments and critique the reasoning of others as they decide whether triangles are identical copies or different triangles (MP3). In the example, the first two figures have angles and adjacent to side . However, in the third figure angle is no longer adjacent to side . Here students can see that a good way to try to make a different triangle with the same 3 measures is to change which sides and angles are adjacent.

Students do not need to memorize how many different kinds of triangles are possible given different combinations of angles and sides, and they do not need to know criteria such as angle-side-angle for determining if two triangles are identical copies.