Final answer:
First-grade math students should use polygons to begin their study of plane shapes. A polygon is a closed figure with straight sides, and students can learn about their properties and how to identify and classify them.
Step-by-step explanation:
To begin their study of plane shapes, first-grade math students should use Polygons.
A polygon is a closed figure with straight sides. Examples of polygons include triangles, squares, pentagons, hexagons, and so on.
Students can learn about the properties of different polygons, such as the number of sides and angles, and how to identify and classify them.